to give an authoritative order: to direct someone to do or stop doing something, with the standing to require it
This is the word for passing along a binding order. It began as a military term, the order relayed down the ranks, and it keeps that flavor throughout the New Testament: a directive from someone with the authority to demand compliance, not a request or a piece of advice.
The range of who does the commanding is wide, yet the act is the same in every case. Jesus charges the disciples and the crowds; the Jewish council and Roman officials issue orders; God himself commands all people everywhere to repent; and above all Paul directs the churches and his coworkers. Whether the speaker is a judge, a teacher, or an apostle, the word marks the message as something to be obeyed.
The orders it introduces run in both directions. Often they are prohibitions, as when Jesus sternly charges those he has healed to tell no one. Just as often they are positive instructions to act. In Paul's letters the word shades toward firm pastoral instruction, telling believers how to live and work, but even there it carries the weight of a command rather than a suggestion.