Text contained within the alt attribute of an HTML image tag. Often called the “alt tag,” the alt text is optional text used to convey information about the image to search engines, screen readers and other non-visual devices. The format is:
<img src="image.jpg" alt="optional alternate text" />
In the above example “image.jpg” is the URL of the image. The “optional alternate text” portion is where the alt text would be inserted. Alt text should provide a brief but accurate description of what is portrayed in the image.
The use of alt text – particularly in images used as links – may improve search rankings by allowing search engines to “read” the content of the image.