An alt attribute, also called alt text or (inaccurately) an alt tag, is a textual alternative to non-text content in software applications or on web pages.
All non-text elements must be labeled with a text equivalent for an application or web page to be Section 508-compliant. In HTML, the alt (alternate) attribute of the <img> (image) tag provides this label, supplemented by the longdesc attribute for more complex images. Many other development platforms use the same term for their image labeling attribute.