the complete equipment of a heavily armed soldier: shield, helmet, breastplate, greaves, sword, and spear taken together as one full set
The whole force of this word is completeness. It is not a general term for armor and not the name of any single piece, but the entire battle kit of a foot soldier gathered into one word: the protective gear and the weapons for attack, nothing left out. Greek writers used it for the full outfit of a Greek soldier and later for the complete equipment of a Roman legionary, and it could even be counted as physical loot stripped from the fallen.
In the New Testament the word is never used of a literal soldier. All three times it is a picture. In one setting it belongs to a strong man who is overpowered: a stronger one strips away the full armor he had trusted in and divides up his goods. In the other it belongs to God, who hands his own complete armor to his people so they can stand firm against a spiritual enemy.
Because the word means the whole set, the point in every use is that no part may be missing. When it is God's armor, defense and attack are both in view; when it is the enemy's, the loss is total once that whole armor is taken away. The one word carries opposite weight in its two New Testament settings, one belonging to the powers of darkness and one to God, yet the plain sense underneath is the same: the complete equipment of a fighter.