Text-to-speech, often abbreviated as TTS, is the process of converting the text of a document to synthetized speech for aural playback. The conversion involves sending the text to a synthetic speech engine to be analyzed and voiced.

Both assistive technologies and reading systems are often capable of generating text-to-speech playback.

In EPUB 3, a top-level content document is an SVG or XHTML document in the spine.

The name "top-level" refers to the fact that these are the primary documents that users encounter as they read the publication. They are not used within other documents (e.g., embedded using the HTML iframe or img elements).