Often the apostles are referred to as "the twelve apostles" - the twelve disciples called by name by Jesus. But Paul called himself and Barnabus apostles. Were Paul and Barnabus really apostles?
The word apostle is a straight transliteration of the Greek apostolos, literally meaning "one who is sent forth." It implies an ambassador or messenger bearing an official message by the authority of someone more powerful than himself. Jesus called twelve Apostles to be his special messengers to all the world.
As we know, Judas, one of the twelve, betrayed him and committed suicide soon after. Matthias was called to replace Judas, which made it clear that the intention was to continue as twelve apostles, replacing the original members as they died. While the Bible doesn't record exactly when or under what circumstances Paul and Barnabas were called as apostles, we do know that there was a lot of persecution of Christians in general and Christian leaders in particular going on at the time. It's reasonable to suppose that they were called as replacements for others of the original twelve.
Specifically, the word Apostle comes from the greek word 'apostolos' which literally means messenger, envoy, or one sent. Apostle essentially means "teacher" as opposed to disciple which means "student".
Specifically in Christianity the Apostles were the 11 disciples who went on to spread the Gospel of Christ after His death and resurrection. Paul and some other 1st generation Christians are included in this term. For most Protestants using the word Apostle as a title only applies to those first generation leaders, although some denominations and schools of theological thought may differ.
I can not speak authoritatively to the use of the word in the Roman Catholic and Eastern Orthodox churches.
Apostle does come from the Greek word
apostello, as noted by Mason Wheeler, and it does indicate one who is sent out.
In the Gospels, the Twelve are most commonly referred to as disciples. They were following the Teacher, learning from Him as His students.
In John 17:18 and Matthew 28:18-20, among other verses, Jesus specifically sends them out and commissions them with the Great Commission.
So, those who were once followers of Jesus became those who were no longer bid to come and follow, but also to go out and proclaim a message.
Even in Matthew 4:19, the pattern for this was established. Jesus said, "Come follow me [be disciples], and I will make you fishers of men [apostles].
This is also the call of everyone who comes to Jesus. We must first and foremost follow Him, but we are also sent out with the Great Commission. Every disciple is called to be an apostle - a sent one.
Apo-stello: those prepared or 'sent-out-of' There is a formal designation for Jesus' original 12, plus Matthias and Paul. (Capital "A" Apostles, if you like)
But there ARE others who are authorised and sent: • James the Lord's brother (on the grounds of 1 Cor 15:7; Gal 1:19) • Paul sets Barnabas as an apostle side by side with himself (1Cor 9:5; Gal 2:9) • As does Luke (in Acts 13:2 f; 14:4,14); • the natural inference from a comparison of 1 Thess 1:1 with 2:6 is that he describes Silvanus and Timothy as "apostles of Christ"; • to the Philippians he mentions Epaphroditus as "your apostolos" interpreted in most bibles as messenger (Phil 2:25)
Apostles can be interpreted, in this manner, as messengers of the gospel. We see Paul (Capital "A" Apostle) and Stephen (not capital "A" apostle) set apart by: "many signs and wonders …regularly done among the people" as we see in 2 Corinthians 12:12 and in Acts 5:12.
There is a casual reference to "Apostles", the disciples that met Jesus personally, whom many regard as elders of the New Testament Church in matters of doctrine (12 plus Paul). And there were other authorised and 'sent-out' disciples who functioned as "apostles" of the church to spread the gospel.