It's very common to hear Christians say "Lord" or "the Lord" in different contexts: everyday conversations, prayer, testimonies, sermons, etc. Some example sentences that come to mind: if the Lord wills, I love the Lord, etc. When Christians say the Lord, whom are they exactly picturing in their heads? Do they mean Jesus (as in "Lord Jesus"), or the Father?
For most Christians "The Lord" or "Lord" will usually mean either God the Father or Jesus. Sometimes they may mean the Holy Spirit or just "God", meaning (for Trinitarians) any or all three of the above.
There are many occasions in the Bible where Jesus is referred to as "Lord", and also God the Father is frequently referred to in that way. (I have not specifically found one where the Spirit is referred to as Lord, but "Spirit of the Lord" is found frequently.) For groups that hold to the Athanasian Creed the equivalence is defined there. " So likewise the Father is Lord; the Son Lord; and the Holy Ghost Lord. And yet not three Lords; but one Lord. "
Seventh Day Adventist Christians - hold the Trinitarian view (I speak from that perspective because it is what I am most familiar with) that where the term Lord is used on its own, which one specifically is being referred to is usually determined by looking at the overall context of the passage of scripture within which the term resides. In some cases, it refers specifically to Jesus, other times the Father, and indeed even the Holy spirit, all three at once, or it could even refer to none of them...(story of Lot with Angels visiting him in Sodom prior to its destruction)
Genesis 19 Angel visiting Lot in Sodom
1Now the two angels came to Sodom in the evening as Lot was sitting at the gate of Sodom. When Lot saw them, he stood up to meet them and [a]bowed down with his face to the ground. 2And he said, “Now behold, my lords, please turn aside into your servant’s house, and spend the night, and wash your feet; then you may rise early and go on your way.”
It is not that Lord defines God, rather, it defines his majesty, honour, and glory... it's a term used even in the secular world to convey reverence, humility, and respect to the intended recipient (for example to an earthly King).
So in SDA denominational beliefs one finds the following Trinitarian position:
- "That God is the Sovereign Creator, upholder, and ruler of the universe, and that He is eternal, omnipotent, omniscient, and omnipresent.
- "That the Godhead, the Trinity, comprises God the Father, Christ the Son, and the Holy Spirit.
- "That Jesus Christ is very God, and that He has existed with the Father from all eternity.
We can then move outside of my denomination's beliefs and look at a very well known Bible passage, 1 John 5:7:
7 For there are three that testify: 8 the Spirit and the water and the blood; and the three are in agreement. (NAS)
At least one Catholic source I have googled has the following statement:
Bishop Anthony B. Taylor delivered this homily on Trinity Sunday, May 26. 
In Mass, when we pray to “Our Lord,” who are we talking to? To God the Father or to Jesus? In the Gospels — prior to his death — when people call Jesus “Lord,” they thought of him like we think of “Our Lady” — a great person but still only human.
It was only after Pentecost that people began to understand that Jesus was God as well as man, such that unlike with Mary and the saints, when we pray to him we do more than just praise him and ask him to intercede for us. We worship him just like we do the Father because Jesus is God too, the second person of the Trinity, whose feast we celebrate today.
So when we pray to “Our Lord” at Mass, who are we talking to — God the Father or Jesus? In Mass — whenever I start with “Let us pray” — the answer is, strictly speaking, to “God the Father,” but since the Trinity is One God, we can’t address the Father without at least implicitly including the Son and Holy Spirit.
I have reference for the Baptists' position on to whom they are assigning the term Lord:
baptism is for believers only - those who can personally declare Jesus as Lord.
The use of the word Lord for Jesus in the Bible comes from the Greek Kyrie.
The wiki for Kyrie has a good write-up about where it's used in the Gospels. And there's a good reason to be confused about it because it is used in by Jesus in a parable about the poor man who knew he was a Sinner and prays, Lord have mercy on me a sinner.
Now, this is an assumption, but I think it is a correct one from experience, adherents to the Eastern Orthodox Church and Greek-speaking Catholic Churches are most likely influenced by their liturgies and hearing 'Lord' in this sense will think of Jesus even though the person in the parable told by Jesus Himself was clearly talking about the God of the Jews who would be recognized, not as a Trinity of persons but as God the Father.
Kyrie Eleison is repeated like a zillion times in their liturgies, as opposed to just 3 in the Latin Rite. But they're apt to repeat "Jesus Christ, Son of the Living God Have Mercy on me a sinner" in clear reference with the person in the parable who repeats "Lord have mercy on me a sinner".
Latin Rite Catholics use it in the same vein, it's literally the only Greek at mass, but in North America and much of Europe, we're influenced by popular culture and Protestantism so we might think of Lord in both ways.
In a hymn like "How great Thou art", I'd imagine "Oh Lord my God" is a reference to God as the Creator of the Universe. Now, a lot of people have an idea the the Trinity is composed of The Creator, The Redeemer and The Sanctifier as titles of the Father, Son and Holy Spirit, so it's easy to understand why God the Father = Lord there. Whether that abides by any theology is not the crux of this question and I won't go there.
But in a popular worship song like Matt Maher's "Lord, I Need You" is definitely about Jesus. Of course, he's a Catholic who a lot of Protestants like so that's confusing and not worth thinking about.
The Newsboys, Lord, is definitely about Jesus too. It never says "Jesus" but "You are the author of knowledge" (Logos, I guess) and "Redeemer".
Matt Redman's "10000 Reasons (Bless the Lord)" is a confusing case, I think it's a reference to the song of the 3 young men roasting in the fire in the Book of Daniel. They were saying how everything blesses the Lord. And there, they're not talking about any revealed Trinity of persons. When you "Bless the Lord" or "Worship the Lord" in modern understand, you're possibly not that worried about that. Songs like this make no distinction between persons of the Trinity.
George Harrison's "My Sweet Lord" is about a Hindu god named Krishna, even with all the Hallelujah's, so.. um, bad example.
So there you have it, "Lord" is a confusing word. I think it means "Master of the Bread" or something like that, in English - so Bread of Life = Jesus. Not that there's anything magic about English, but it's pretty cool.
"Lord" was used by Jesus when speaking to His Father:
At that time Jesus declared, “I thank you, Father, Lord of heaven and earth, that you have hidden these things from the wise and understanding and revealed them to little children (Matthew 11:25, also Luke 10:21) [ESV]
As the term means "he to whom a person or thing belongs, about which he has power of deciding; master..." it would be appropriate for Jesus to address His Father as Lord. However, after His death and resurrection, that condition changed:
And Jesus came and said to them, “All authority in heaven and on earth has been given to me. (Matthew 28:18)
There will come a time when there will be another change (cf. 1 Corinthians 15:24-28), but currently all authority rests with Jesus, which is best stated in using Lord:
yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist. (1 Corinthians 8:6)
Moreover, addressing the Father as "Lord" is misleading as to the current relationship between the individual Christian and the Father:
For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” (Romans 8:15)
How individual Christians apply the term "Lord" today may vary, yet the NT instructs the earliest Christians to say "Abba Father" and "Lord Jesus Christ."
Oscar Cullmann describes the title of Lord as it was used by the early Church:
In designating Jesus as the Kyrios the first Christians declared that he is not only a part of divine Heilsgeschichte in the past, nor just the the object of future hope, but a living reality in the present - so alive that he can enter into fellowship with us now, so alive that the believer prays to him, and the Church appeals to him in worship, to bring their prayers before God the Father and make them effective. Both the individual Christian and the gathered Church experience in faith the fact that Jesus lives and continues his work. The Church as the Body of Christ is founded on faith in the exalted Christ who still intervenes in earthly events. The first Christians expressed this deep conviction in their confession of faith Kyrios Jesus, 'Jesus is Lord.'
"Lord" was a most significant title applied to Jesus. As Cullmann says, "The designation of Jesus as Kyrios has the further consequence that actually all the titles of honour for God himself (with the exception of 'Father') may be transferred to Jesus."
1. Oscar Cullmann, The Christology of the New Testament, Revised Edition, translated by Shirley C. Guthrie and Charles A. M. Hall, The Westminister Press, 1963, p. 195
2. Ibid., p. 236-7