elusive

Etymology

From Latin elusus past participle of eludo (“to parry a blow, to deceive”).

adj

1. Evading capture, comprehension or remembrance.
   The elusive criminal was arrested
2. Difficult to make precise.
   A precise definition of diarrhea is elusive 2010, Robbins & Cotran Pathologic Basis of Disease
   Charley chased the elusive idea through all the nooks and crannies of his drowning consciousness. 1910, Jack London, chapter 6, in Lost Face, archived from the original on 2011-04-14
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, Scott Roederer, Birding: Rocky Mountain National Park, page 93