Apostolos means one who has been sent.
Yes.
This this is the lexical definition. But in the context of the ministry of Christ and subsequently that of the apostles, it goes a lot further.
1. An apostle of Christ was a man who was specifically chosen out of his disciples to become one who would have apostolic authority and also perform miracles.
2. The apostles of Christ were primarily Galileans, and Judas was deliberately selected to be the one who would betray Christ.
3. The names of the twelve apostles were:
Simon Peter (also called Cephas) and Andrew his brother,
James and John the sons of Zebedee,
Philip and Bartholomew (also called Nathanael),
Thomas and Matthew (also called Levi),
James the son of Alphaeus and Lebbaeus (his brother, also called Thaddeus and Judas),
Simon the Canaanite, and Judas Iscariot (Judas replaced by Paul).
You could say that this list gives the names in their order of importance. There were three sets of brothers, and there were also family relationships.
4. Christ first sent these 12 to preach the Gospel of the Kingdom, and to perform miracles. (Later He sent seventy others).
5.
They were companions of Christ throughout his ministry, but forsook Him after He was taken prisoner (which He expected since it was prophesied).
So this is another criterion.
6. After His resurrection, Christ appeared to ten of them, and later on to Thomas who was absent.
So another criterion for being an apostle is that they were eye-witnesses to the Risen Savior and he had appeared to them personally (this also applied to Paul). And since Revelation 21:14 tells us that there are only twelve apostles of the Lamb. Anyone else who was incidentally called an apostle was because he was a sent one accompanying an apostle.
7. Then Christ met them at some of them at the Sea of Tiberias, following which He remained in Galilee for 40 days teaching his apostles.
8. When He ascended He told them to wait in Jerusalem and that they would receive power to preach the Gospel, beginning in Jerusalem, then Judea, then Samaria, and then to the uttermost parts of the world.
9. On the Day of Pentecost, they were all gathered together and received the Holy Spirit with speaking in tongues. Peter preached and many Jews were converted.
10. The apostles remained in Jerusalem for a while, then scattered after the persecution of Christians.
11. While all are not seen in Acts, some are shown to have done miracles during their ministry.
Another important criteria for being an apostle was that they performed "signs, wonders, and miracles.
How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him; God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost, according to his own will? (Heb 2:3,4) And Paul established His apostleship by saying that he too had performed these miracles.
Truly the signs of an apostle were wrought among you in all patience, in signs, and wonders, and mighty deeds. (2 Cor 12:12)
12.
The apostles and prophets were FOUNDATIONAL to the Church, and that is another criterion.
And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone; In whom all the building fitly framed together groweth unto an holy temple in the Lord: In whom ye also are builded together for an habitation of God through the Spirit. (Eph 2:20-22)
For us today, that foundation is found in the divinely inspired writings of the apostles in the NT. Peter, Paul, and John were all apostle-prophets. And John made it clear that all genuine prophecies had come to an end. Paul made it clear that all genuine tongues had also come to an end.