An online advertising tactic where a business delivers their ad to the people that have previously been to the website, ideally with the same or similar product that a person has checked. Since there are very few people who make a purchase on the first visit, retargeting is a powerful way to convert website visitors into customers. The ad should be relevant, i.e. feature the very product that the user viewed before.
Advertisers use cookies and pixels to identify which online users have visited their website to target them in the ads later. For example, if you check out a pair of sneakers on a Nike website, but don't buy them, you will most probably see an ad for these sneakers someplace online.