elusive

Evading capture, comprehension or remembrance.

The elusive criminal was arrested

The temporary elusive goal / To reach the solace, to feed once more upon the synthetic reaper of loss / No matter the outcome, the cost

Difficult to make precise.

A precise definition of diarrhea is elusive

Charley chased the elusive idea through all the nooks and crannies of his drowning consciousness.

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.