Refers to strong influence or authority, often in political or organizational contexts. Somewhat formal and less common in everyday speech than synonyms like 勢力 or 影響力.
His influence within the party is immense.
The general wielded great power.
Compound of 勢 (power, force) and 威 (authority, dignity). The exact historical derivation is uncertain, but the combination naturally conveys strong, authoritative influence.