na-adjective
Describes an attitude, posture, or measure taken to protect oneself or one's position, often in contrast to being proactive or offensive.
彼の態度はいつも防衛的だ。
His attitude is always defensive.
防衛的な戦略を取る必要がある。
We need to adopt a defensive strategy.
防御的 is a near-synonym also meaning 'defensive', but 防衛的 often implies a broader, more strategic or psychological defense, while 防御的 can be more physical or protective.
Compound of 防衛 (defense) + 的 (-like, -ive).