Entry 2 senses · 2 variants Webster, 1913 Necessarian · Nec·es·sa·ri·an · IPA /ˌnɛsəˈsɛɹi.ən/ n. a. 01 n. An advocate of the doctrine of philosophical necessity; a necessitarian. 1. An advocate of the doctrine of philosophical necessity; a necessitarian. 02 a. Of or pertaining to necessarianism. 1. Of or pertaining to necessarianism.