a loss; a failure; a mistake
A single mistake or failure, often used in the context of learning from errors or in contrast with success. Common in set phrases like 一失を顧みる.
He reflected on his one mistake and prepared for the next match.
We can't just laugh off a single failure.
gaining some and losing some; advantages and disadvantages
a slip by a wise person; a mistake even the cautious make
even a wise man sometimes errs; Homer sometimes nods
Compound of 一 (one) and 失 (loss, mistake). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for counting errors.