Influence, Force
noun
Your force and power combine together to create an influence (or a force).
せいりょく
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
世界の二大勢力と言えば、どの国とどの国を思いうかべますか？
When you think of the two major powers in the world, which two countries do you think of?
台風の勢力は、夜にかけて次第に弱まっていきました。
The typhoon's strength gradually weakened over night.
その武装勢力は、急速に勢力を増大している。
The armed insurgents are rapidly increasing their influence.