If your grandfather worked his whole life in a circus and inspired you to wear clown shoes, you could describe your grandfather as very influential in your life. Something that exerts influence or power can be called influential.

You could read a deeply meaningful book that becomes influential to you, or there might be an important person in your life who is strongly influential over various decisions you make. The Latin word influentem, "flowing in," is the root of the Old French influence, which described the power was believed to flow from the stars.

Definitions of influential
  1. adjective
    having or exercising influence or power
    “an influential newspaper”
    influential leadership for peace”
    authoritative, important
    having authority or ascendancy or influence
    potent, powerful
    having great influence
    exerting influence by reason of high status or prestige
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    not influential
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