Pronunciation: (rOO'du-munt),[key] —n. 1. Usually, rudiments. a. the elements or first principles of a subject: the rudiments of grammar. b. a mere beginning, first slight appearance, or undeveloped or imperfect form of something: the rudiments of a plan. 2. Biol.an organ or part incompletely developed in size or structure, as one in an embryonic stage, one arrested in growth, or one with no functional activity, as a vestige.