elusive

adjective3

1. •
   Evading capture, comprehension or remembrance.

   • The elusive criminal was arrested
2. •
   Difficult to make precise.

   • A precise definition of diarrhea is elusive — 2010
3. •
   Rarely seen.

   • While you're sniffing the trunks of the ponderosas to see if they're butterscotch, vanilla, strawberry, or the elusive chocolate variety, watch for Brown Creepers, an elusive variety of bird. — 2002