rudiment

Noun

• 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
    

• the elementary stages of any subject (usually plural)
  • he mastered only the rudiments of geometry
    

Related words

• Synonyms: 
  first rudiment , first principle , alphabet , ABC , ABC's , ABCs