Definitions for "Alternate text"
a word or phrase that is displayed when a user has image loading disabled in their browser or when a user abandons a page by hitting "stop" in their browser prior to the transfer of all images. Also appears as “balloon text” when a user lets their mouse
descriptive text that is meant to be displayed by the browser when it cannot locate a specific image or other piece of content, or when the user has configured the browser to turn off graphics images. Some search engines will index the contents of ALT text and use it when determining relevancy and ranking.
A description of a hyperlink or image, put in by the author of a Web page, that pops up when you move the mouse pointer over the hyperlink or image.