tern vs trefoil

tern

noun
  • A thing with three components; a set of three things. 

  • Any of various seabirds of the subfamily Sternidae (of the family Laridae) that are similar to gulls but are smaller and have a forked tail. 

adj
  • Consisting of three components; ternate, threefold, triple. 

trefoil

noun
  • A symbol having the shape of such leaves, especially when used as an architectural ornament. 

  • Any of several plants of the pea family, having compound, trifoliate leaves; especially one of the genus Trifolium. 

How often have the words tern and trefoil occurred in a corpus of books? (source: Google Ngram Viewer )