WOTD: Elusive

e·lu·sive   /ɪˈlusɪv/ [ih-loo-siv]

hard to express or define

adjective
1. eluding clear perception or complete mental grasp; hard to express or define: an elusive concept.
2. cleverly or skillfully evasive: a fish too elusive to catch.

Synonyms
2. tricky, slippery, shifty; puzzling, baffling.

source: http://dictionary.reference.com/browse/elusive