elusive

adj

1.
difficult to describe
Derived from:
elude elusiveness
2.
skillful at eluding capture
Derived from:
elude elusiveness
3.
difficult to detect or grasp by the mind or analyze
subtle
Derived from:
elude elusiveness
4.
making great mental demands; hard to comprehend or solve or believe
baffling knotty problematic problematical tough