A 1 x 1 pixel graphic that tracks various events on a webpage. It's an invisible image file placed on a website that is used to pass information, such as a cookie, to a server. Pixels can be used to determine touchpoints with a brand across different platforms.
Tracking pixels help inform advertisers on their site engagement by providing insight into where users are coming from (i.e. an organic or paid search, referral website, social ad), how users engage once on the site (i.e. time on site, bounce rate, if they add to cart or purchase) and build targeting audiences based on site engagement.
Also pixel can defined as a small unit of measurement of an image, with thousands of individual pixels forming what the eye sees as one smooth image.