Entry 1 sense Webster, 1913 Principia /prĭn-sĭp'-ē-ə/ · Prin·cip·i·a · IPA /prɪnsˈɪpiːə/ 01 n. pl. First principles; fundamental beginnings; elements; as. Newton's Principia. 1. First principles; fundamental beginnings; elements; as. Newton's Principia.