From apo (away from) and stellō (to set, send), meaning to send someone forth on a mission with authority and purpose. Distinguished from pempō (G3992, simply 'to send'), apostellō emphasizes the commission and authority of the one sent. It is the root of apostolos (apostle — one who is sent).
The theology of sending permeates the NT. The Father sent the Son (John 3:17; 1 John 4:9-10); the Son sends the Spirit (John 15:26); and the risen Christ sends His disciples: 'As my Father hath sent me, even so send I you' (John 20:21). This chain of sending establishes the missio Dei — God's mission flows from the Trinity outward into the world through commissioned agents. An apostolos carries the sender's authority — 'he that receiveth whomsoever I send receiveth me' (John 13:20). The Great Commission is the climactic apostellō: the risen Lord dispatching His church into all nations with His authority and His presence (Matt 28:18-20). Every believer participates in this divine sending.