The HTML attribute that specifies alternative text for an image is `alt`.
An image's `alt` value provides a concise text replacement when the image cannot be displayed and gives assistive technologies information about the image's purpose. Screen readers commonly announce it, while text-only browsers can show it in place of the missing image. Good alternative text describes the image's meaning in context, rather than mechanically listing every visible detail.
The attribute has accessibility roots: it was introduced in an HTML 1.2 draft in 1993, partly to support text-based browsers. In HTML 4.01, alternative text was required for `img` and `area` elements. For a purely decorative image, `alt=""` is usually appropriate so a screen reader can skip it; omitting the attribute altogether can create confusion.
`alt` is not the same as `title`. The latter may provide advisory information and is not a dependable substitute for accessible alternative text. `src` identifies the image file to load, `href` identifies a hyperlink destination, and neither describes the image. Concise, meaningful alt text can also help image indexing, but accessibility and communication—not keyword stuffing—should guide its wording.