From apostellō (to send out) — one sent with authority as an official representative. In secular Greek, it could mean an envoy or ambassador. The NT uses it primarily for the twelve disciples chosen by Jesus and for Paul, who was called directly by the risen Christ.
The apostoloi were not self-appointed but divinely commissioned — 'sent ones' who carried the authority of the sender. Their unique role was foundational: the church is built on 'the foundation of the apostles and prophets' (Ephesians 2:20). Apostolic authority meant their witness to the resurrection was the bedrock of Christian proclamation. The concept of being 'sent' runs throughout John's Gospel — the Father sent the Son, the Son sends the apostles.