Alt text is the alternative text attribute on <img> elements (alt="..."). It describes the image for users who cannot see it — screen-reader users, users on slow connections where the image fails to load, search engine crawlers indexing the image. Required by WCAG 2.1 accessibility standards.
Good alt text is descriptive but concise — 5-15 words capturing what is shown and why it matters. "Dashboard screenshot showing the audit results panel with a 92/100 AEO score" beats "screenshot of dashboard" or "image1.png". For purely decorative images (visual flourishes, dividers), use alt="" to signal to screen readers that the image can be skipped.
For SEO, alt text is the primary signal Google uses to understand image content. Images with descriptive alt text rank in Google Images for relevant queries, drive traffic via Image Search, and reinforce the page's topical relevance. Missing alt text is one of the most common technical SEO failures.
For AEO, alt text matters less directly because AI engines focus on text content. But it contributes to the page's overall accessibility and quality signal — sites with consistent alt-text coverage signal editorial maturity, which AI engines weigh as a quality factor.