Simple Definition:

An influencer is someone who has the ability to affect or change the opinions, behaviors, and purchasing decisions of others, especially on social media platforms.

For example, a popular fashion influencer on Instagram can influence their followers to buy certain clothing brands or try new fashion trends.

Very Simple Definition:

An influencer is someone who can make other people think, act, or buy things.

For example, a famous person on TikTok who tells their fans to use a particular shampoo can make them want to buy and use that shampoo too.