Lasting for a very short time; brief or fleeting.
From Latin 'momentum' (movement, moment in time) which comes from 'movimentum' (motion). The suffix '-ary' means relating to or characterized by. The word evolved to mean 'lasting only a moment' in the 1500s.
The word 'momentum' (which means force and speed) and 'momentary' (which means brief) come from the same Latin root! That's because to ancient thinkers, both a moment in time and a movement through space felt equally fleeting and hard to hold onto.
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