rudiment in WordNet English dictionary

1. the elementary stages of any subject (usually plural)
   "he mastered only the rudiments of geometry"
   rudiment  first rudiment  first principle  alphabet  ABC  ABC's  ABCs 
   
2. the remains of a body part that was functional at an earlier stage of life
   "Meckel's diverticulum is the rudiment of the embryonic yolk sac"
   rudiment