Alternate text (alt text) is text which describes the meaning and context of a visual item in a digital setting, such as on an app or web page. When screen readers reach digital content with alt text, they will read the alt text aloud, allowing people to better understand what is on the screen. Well-written, descriptive alt text improves clarity and user experience.
The alt text shouldn’t be longer than a sentence or two—most of the time a few thoughtfully selected words will do. Consider what is important about an image. For example, important context might be the setting, the emotions on people's faces, the colours, or the relative sizes.
If the image is purely decorative but not informative (e.g. stylistic elements), simply select the ‘Mark as decorative’ checkbox when the Alt Text pane is open. The text entry field becomes grayed out. People using screen readers will hear these are decorative, so they know they aren’t missing any important information.
Definition adapted from Microsoft Office.
You can find more information on how to write an alternate text in our additional resources section.