battle: the clash of armed fighting itself, whether between armies or aimed at a single person
This is a poet's word for battle. Outside one line in 2 Samuel it turns up only in Job, Psalms, Ecclesiastes, and Zechariah, while the ordinary Hebrew word for war (מִלְחָמָה) does all the everyday work in narrative and law. Twice the two sit in the same line, so a Hebrew reader hears a deliberate pairing where an English reader sees only two words for the same thing: "the day of battle and war" in Job 38:23, and "who trains my hands for war, my fingers for battle" in Psalm 144:1.
What holds the uses together is the moment of contact. The word does not describe the long political condition of being at war so much as the fighting itself: the day you march into it (2 Samuel 17:11), the day the archers of Ephraim turned and ran (Psalm 78:9), the day God himself goes out to fight (Zechariah 14:3). From there it stretches easily to the gear and the habit of fighting, so wisdom can outperform "weapons of war" in Ecclesiastes 9:18, and whole peoples can be described as those who simply "delight in war" in Psalm 68:30.
Psalm 55 pushes it one step further, to a fight with no army in it. The writer is rescued "from the battle that was against me" (verse 18), and the man who betrayed him spoke words smoother than butter while "war was in his heart" (verse 21). One word covers a national campaign and a private grudge, which is why the psalm can describe a friend's treachery in the vocabulary of the battlefield.