also: いきよい
noun
Describes physical force, liveliness, or energetic spirit, often in expressions such as 勢いがある and 勢いが強い.
火の勢いが急に強くなった。
The force of the fire suddenly grew stronger.
彼女の演説
Her speech had energy.
noun
Used for the power or influence held by a person, family, regime, organization, or other force; somewhat common in historical or political descriptions.
そのころ、平氏の勢いは非常に強かった。
At that time, the Taira clan's power was extremely strong.
noun
Covers the forward drive of events or feelings, including doing something on impulse in the phrase 勢いで and riding momentum in 勢いに乗る.
チームは連勝の勢いに乗って決勝まで進んだ。
The team rode the momentum of its winning streak all the way to the final.
その場の勢いで高い時計を買ってしまった。
I got carried away in the moment and bought an expensive watch.
adverb
inevitably; naturally; necessarily
Adverbial use meaning that something follows naturally or inevitably from the circumstances; it is more written or formal than everyday expressions such as どうしても or 結局.
議論が長引けば、勢い結論は先送りになる。
If the discussion drags on, the conclusion will inevitably be postponed.