Elusive

e·lu·sive (-lsv, -zv)
adj.

1. Tending to elude capture, perception, comprehension, or memory: “an invisible cabal of conspirators, each more elusive than the archterrorist [himself]” (David Kline).
2. Difficult to define or describe: “Failures are more finely etched in our minds than triumphs, and success is an elusive, if not mythic, goal in our demanding society” (Hugh Drummond).