A psychological phenomenon in which people assume the actions of others accurately reflect the correct behavior in a social situation.

Social proof often plays out in marketing by creating a sense of validation. It is seen when a company or brand emphasizes that X number of other people bought or liked this product, so you should too.

By reflecting on the way other's have acted, you're more likely to believe their actions are the correct behavior -- and, therefore, you should buy or like the product, too.