1) Ambiguous 2) Baffling 3) Cagey 4) Close to unattainable 5) Difficult to catch 6) Difficult to describe 7) Difficult to find 8) Difficult to grasp 9) Difficult to recall 10) Eely 11) Equivocal 12) Hard to catch 13) Hard to define 14) Hard to describe 15) Hard to find 16) Hard to get 17) Hard to grasp 1) Baffling 2) Eely 3) Hardtopindown 4) Insubstantial 5) Scarce 6) Slippery 7) Slippery as an eel 8) Sly 9) Subtle 10) Unidentifiable 
• (a.) Tending to elude; using arts or deception to escape; adroitly escaping or evading; eluding the grasp; fallacious.
[Adjective] Hard to grasp.
E·lu'sive adjective Tending to elude; using arts or deception to escape; adroitly escaping or evading; eluding the grasp; fallacious. « Elusive of the bridal day, she gives Fond hopes to all, and all with hopes deceives.» Pope. -- 
[adj] - be difficult to detect or grasp by the mind 2. [adj] - difficult to describe 3. [adj] - skillful at eluding capture
elusive Tending to elude perception or comprehension; difficult to describe.
evasive adjective skillful at eluding capture; `a cabal of conspirators, each more elusive than the archterrorist`- David Kline
eluding clear perception or complete mental grasp; hard to express or define: an elusive concept. · cleverly or skillfully evasive:too elusive to catch. a fish
[SAT terms] skillful at evading capture[Literary terms] skillful at evading capture