In HTML, an attribute of the img tag that allows persnickety browsers to display text in place of the image, or to indicate the content of the image to the visually impaired and search engine spiders.
In the event that an image displayed on a web page displays text or other information, the Alt attribute should be set with descriptive text so that persnickety browsers, visually impaired people, and search engines can read the description. A lot of SEO spammers get themselves banned from search engines by keyword stuffing their Alt attributes.
Some accessibility nuts like to ruin a webmasters day by insisting that every blasted, non-informative image must have an "Alt tag". Some of us cave in to their nonsense by using NiDE, but the truth is, there is no such thing as an Alt tag. Alt is an attribute of the img tag.