hair's breadth; hair
Literally 'one hair'. In modern Japanese, it is almost exclusively used in the set phrase 危機一髪 (ききいっぱつ), meaning 'a close call' or 'by a hair's breadth'. The standalone noun use is rare and mostly found in classical or literary contexts.
I was saved by a hair's breadth.
「一髪」という言葉は、現代ではほとんど「危機一髪」の中でしか見かけない。
The word 一髪 is almost never seen outside of 危機一髪 in modern Japanese.
hair's breadth; close call; by a hair
From the Chinese idiom 一髪千鈞 (yī fà qiān jūn), literally 'a thousand jun hanging by a single hair', indicating a precarious situation. The Japanese form 危機一髪 is a later adaptation.