What is the word which means that the Father, the Son, and the Holy Ghost cannot be separated? When we receive Holy Communion, the Host contains Jesus, the Father, the Holy Ghost, and all that God created because God is omnipresent in all creation, so everything comes with the Host, most importantly, Jesus' Body, Blood, Soul, and Divinity! But there is a word for this and I cannot remember what it is. Can you please help?
If I understand your question correctly, then that word is the mystery of the Holy Trinity, a mystery of faith. It said that there is only one God, but in three persons (yes, "persons" is the right term)
There is an analogy provided on Catholic.com, which is really good in giving something that we can relate to, but no analogy is perfect, and this one is no exception. Here is the analogy:
"In God we see the Father—the “being one” and first principal of life in the Godhead—the Son—the “knowing one”—the Word who proceeds from the Father—and the Holy Spirit—the “willing one”—the bond of love between the Father and Son who proceeds as love from the Father and Son. These “three” do not “equal” one if we are trying to say 3=1 mathematically. These three are distinct realities, relationally speaking, just as my own being, knowing, and willing are three distinct realities in me. Yet, in both God and man these three relationally distinct realities subsist in one being.
As St. Augustine points out, we can never know God or understand God completely through this or any analogy, but it can help us to understand how you can have relational distinctions within one being. And we can see this is reasonable.
The weakness inherent here—there are weaknesses in all analogies with reference to God—is that our knowing, being, and willing are not each infinite and co-extensive as the persons of God are. They subsist in one being in us, but they are not persons."
Hope that helped