Apostle is an English transliteration of the Greek apostolos, which means "one who is sent out." In ancient Greek, an apostle might be any person "sent out" to deliver news - messengers and envoys, for example - and perhaps carry out other instructions. Via the New Testament, apostle acquired a more specific usage and now refers to one of the elect original disciples of Jesus. Apostolic lists in the New Testament all have 12 names, but not all the same names.
Christians see the apostles as a connection between the living Jesus, the resurrected Jesus, and the Christian church that developed after Jesus ascended to heaven. The apostles were witnesses to Jesus' life, recipients of Jesus' teachings, witnesses to appearances of the resurrected Jesus, and recipients of the wisdom of the Holy Spirit. They were authorities on what Jesus taught, intended, and desired. Many Christian churches today base the authority of religious leaders on alleged connections to the original apostles.