Refers to a constitution that renounces war and the maintenance of military forces; in Japanese context, almost always refers to the post-WWII Constitution of Japan (especially Article 9).
Article 9 of the Japanese Constitution, which is the core pacifist clause; 平和主義憲法 is often used synonymously with the constitution containing this article.
Etymology
Compound of 平和主義 (pacifism) and 憲法 (constitution).