An alt attribute, short for alternative attribute, is a descriptive text or phrase that is added to an HTML code to provide alternative information about an image or other non-textual element on a webpage. This attribute is used to improve accessibility for individuals with visual impairments, as it allows screen readers to accurately describe the content of an image. Additionally, alt attributes can also benefit search engine optimization by providing relevant keywords for search engines to index. In essence, alt attributes serve as a bridge between visual and textual content, making web content more inclusive and user-friendly.