noun

1. a first principle, element, or fundamental, as of a subject to be learned: usually used in pl. the rudiments of physics
2. a first slight beginning or appearance, or undeveloped form or stage, of something: usually used in pl.
3. Biol. an incompletely developed organ or part; specif., a vestigial organ or part with no functional activity; vestige

Etymology: L rudimentum < rudis: see rude