This is the 613th most frequent Japanese word.
防衛
Defense.
In this context, "防衛" is used metaphorically to mean the act of defending an argument or viewpoint.
彼女の意見に対する防衛はとても論理的でした。
Her defense of her opinion was very logical.
Here, "防衛" describes a natural instinct related to self-preservation or protection among living organisms.
防衛本能は自然界の生物に普遍的なものです。
The defense instinct is universal among living organisms in the natural world.
Here, "防衛" is used in its literal sense, meaning defense, referring to the action of protecting a country or place.