na-adjective
Describes something intended to continue indefinitely without change. Often used in formal or technical contexts such as law, policy, or infrastructure.
恒久的な平和を築くのは難しい。
Building a permanent peace is difficult.
この施
It was decided that this facility would be permanently closed.
一時的 means temporary or transient, the direct opposite of 恒久的.
Compound of 恒久 (こうきゅう, 'permanence') + 的 (てき, '-like, -al'). 恒久 is itself composed of 恒 ('constant, always') and 久 ('long time').