an order given by someone with the standing to be obeyed: a charge that binds rather than advises
This word names an order, and its whole force comes from who gives it. The speaker has the right to be obeyed, so what is said is not a suggestion the hearer weighs and then accepts or declines; it settles the matter. Greek writers of the period used the set phrase translated "by command of" on dedications and public notices to name the god or the official who had ordered a thing done, and Paul uses it the same way.
Six of its seven appearances in the New Testament turn on that authority, and they fall into two groups that mirror each other. Three of them put God's own charge behind Paul's work: the long hidden message is now made known, and Paul is made an apostle and trusted with the preaching, at God's command rather than on his own initiative. The other three do the reverse. Twice Paul tells his readers that what he is saying is not one of these, and once he says that on a question about the unmarried he has no such order from the Lord at all, so what he offers instead is his considered judgment (γνώμη). Setting the two side by side is how he grades how binding his own words are.
The last appearance stretches the word one step further, from the order itself to the authority to give one. Titus is told to teach with all of it and to let nobody look down on him, so what is in view is the weight he speaks with, not any single instruction.
That range is why this word is not simply a synonym for the more common word for a commandment (ἐντολή), which names a standing precept one lives under. This one points to a particular charge and to the standing of whoever issues it.