adj.
 not known or seen or not meant to be known or seen by others. 
▸fond of having or keeping secrets; secretive.
n.
   - something kept or meant to be kept secret. 
▸something not properly understood; a mystery:
 the secrets of the universe.
 - a way of achieving something, especially one that is not commonly known:
 the secret of a happy marriage is compromise.
 - a prayer said quietly by the priest after the offertory in a Roman Catholic Mass.
 
 
  Derivative
  
  Etymology
  ME: from OFr., from L. secretus ‘separate, set apart’, from secernere, from se- ‘apart’ + cernere ‘sift’.