An image’s text alternative. It communicates relevant meaning or function when an image cannot be seen.
Alt text is the text alternative provided for an image, usually through the HTML alt attribute. Its job is to convey the relevant information or function when the image cannot be seen. The right wording depends on what the image contributes in that particular context.
An informative photograph may need a brief description of its important content. A purely decorative image can use an empty alt attribute so it does not add unnecessary noise. If an image acts as a button or link, its alternative should describe the action or destination rather than only its appearance.
Do not use alt text as a place to repeat search phrases unrelated to the image. For a project photograph, describe what the photograph actually shows. Avoid adding an unverified location, material or outcome just because it would be commercially useful. Complex charts may also need a fuller explanation in nearby text; a short attribute alone may not convey all their information.
Takeaway: Write image alternatives for the reader who cannot see the image. Accurate, context-specific descriptions improve access to useful information and avoid turning image fields into keyword lists.
Example
A furniture maker shows a close-up of a chair’s adjustable armrest. “Chair armrest with height-adjustment lever” conveys the visible feature. A decorative background pattern needs different treatment, while a magnifying-glass image used as a search button should communicate “Search.” The purpose of each image determines the appropriate alternative.
What to check
Review images in their page context rather than generating one description for every reuse. Check functional images, informative photos and decorative elements separately. Google also advises useful image context and avoiding keyword stuffing in alt text; accessibility remains the starting point.
For more detail, see Images tutorial: W3C Web Accessibility Initiative.