Voiceovers: offscreen, offstage or otherwise unseen narrations, announcements or vocal utterances — voice — usually (but not necessarily) heard over visual images.

These are all examples of voice overs:

• radio and TV station identifications,
• documentary narrations,
• voicemail greetings,
• web voice-assist prompts,
• corporate video narrations,
• training narrations,
• radio commercials,
• off-camera television commercials,
• medical narrations,
• audio book narrations,
• Mary Alice Young‘s insights on Desperate Housewives, and
• the most famous line in Citizen Kane (because it was added to the scene with enhanced sound effects in post production — after the scene was shot).

The term voice over originally came from the film industry. It was the term used to designate on the movie script the off-camera voice heard over the onscreen action.