Definition: An alt tag, also known as "alt attribute" and "alt description," is an HTML attribute applied to image tags to provide a text alternative for search engines. Applying images to alt tags such as product photos can positively impact an ecommerce store's search engine rankings.
Search engines and other robots cannot interpret images, but images can play a crucial part in how people interpret a particular web page. Alt tags solve for this by providing text which is read by the search engines. When Googlebot or other search engine crawlers inspect a page, images with properly formatted alt text contribute to how the page is indexed and where it ranks.
Alt tags are also useful for users viewing a webpage on screen readers or browsers that can't process images.
Alt text is contained within the image tag: <img src="myimage.png" alt="nike_air_zoom" />.