elusive

Hard to catch, find, understand, or remember.

In other words: hard to catch, hard to grasp, hard to remember

• The suspect remained elusive for weeks.

Vague or difficult to define precisely.

In other words: vague, indefinite, hard to pin down

• The exact cause of the problem is still elusive.

Rarely seen or encountered.

In other words: rare, uncommon, seldom seen

• The elusive bird was spotted only once that season.