Alt Attribute
The alt attribute (alt) is an HTML attribute for images that provides alternative text, shown when an image fails to load and read aloud by screen readers; it is required by WCAG success criterion 1.1.1 (Non-text Content).
In practice
In practice, alt text should describe the image content precisely and briefly, without keyword stuffing and without repeating an adjacent caption. Purely decorative images get an empty alt="" instead, so that screen readers skip over them. Context matters too: the same image may need different information in an online shop than in an editorial article. The SEO perspective on alt text, including how it interacts with image search and keywords, is covered in the separate glossary entry seo-content/alt-text.