nontrivial; non-obvious
Technical term used in mathematics, logic, and formal contexts to describe a solution, proof, or fact that is not immediately apparent or requires significant reasoning.
The proof of this theorem relies on a nontrivial lemma.
It looks simple at first glance, but it is actually a non-obvious problem.
Compound of 非 (hi, 'non-') and 自明 (jimei, 'self-evident'). The word 自明 is itself a Sino-Japanese compound of 自 (self) and 明 (clear, obvious).