Meaning

noun (military) military action or resources protecting a country against potential enemies

noun protection from harm

noun (sports) the team that is trying to prevent the other team from scoring

originFrom Middle English defens, defense, from Old French defens, defense, from Late Latin dēfēnsa (“protection”). Displaced native Old English bewering. The verb is from the noun.

Etymology from Wiktionary, CC BY-SA 3.0