what a person has in mind: the thoughts someone forms, and so also the thinking mind that forms them.
Greek made this word from the verb for thinking, with an ending that names the product of an action. So it does not point to the mind as an organ or a faculty; it points to the mind's output, the thing actually thought. Paul is the only New Testament writer who uses it, six times in all, five of them in his second letter to Corinth, and every time in a setting where what people are thinking is either under attack or under guard.
Because a person's thoughts are the whole content of their thinking, the plural slides easily into standing for the thinking person. Thoughts can be hardened, blinded by the god of this age, corrupted away from wholehearted devotion, or kept safe by the peace of God. English versions usually say "minds" in those verses, and that is fair, but the picture underneath is of the thoughts themselves rather than of a mental machine that makes them.
The same word also narrows in two directions. Taken one at a time, a thought is something concrete enough to be captured, which is how Paul can picture every proud thought being taken prisoner and marched into obedience to Christ. Aimed at a target, a thought becomes a plan, which is why he can speak of what Satan has in mind as his designs. The word itself stays neutral in both cases; the menace in the second belongs to the one doing the thinking, not to the vocabulary.
A companion word makes the shape clearer. Where νοῦς is the mind itself, the understanding a person thinks with, this word is what that mind turns out. Philippians 4:7 sets the two side by side: the peace of God surpasses all understanding, and then stands guard over hearts and thoughts.