Refers to a single item, event, or matter, often in contrast to others. Common in set phrases like 一事が万事 (one thing indicates everything).
From his behavior, one thing tells you everything.
I want you to remember just this one thing.
ne bis in idem; double jeopardy prohibition
principle of not resuming debate on a bill voted down in the same Diet session
Compound of 一 (one) and 事 (thing, matter). The reading いちじ is the standard on'yomi combination.