Momentary means lasting only an instant. Gone before you have fully noticed it.
It describes lapses, pauses, hesitations and flickers of feeling, so a momentary lapse of concentration is the standard explanation for a great many accidents. The word is about duration only, not importance, since a momentary decision can change everything. Take care with momentarily, which in British English means for a moment and in American English usually means in a moment, and that difference has caused real confusion on aeroplanes.
1adjectivemomentaryLasting for a very short time.
“There was a momentary silence before anybody spoke.”
Momentary is of the moment. Latin mōmentum meant a short time or an instant, so the word is that noun with an adjective ending and nothing else.
Latin
mōmentārius — of brief duration
Middle English
momentare
English, today
momentary
Built from
From Middle English momentare, from Latin mōmentārius (“of brief duration”), from mōmentum (“a short time, an instant”). by surface analysis, moment + -ary.
Etymology from Wiktionary, CC BY-SA