Voice Over

(Television, Film) recorded narration or voice added after the completion of a film; film or video sequence containing an off-screen narrator; voice of an off-screen character or narrator

The term voice-over refers to a production technique where a disembodied voice is broadcast live or pre-recorded in radio, television, film, theater and/or presentation. The voice-over may be spoken by someone who also appears on-screen in other segments (e.g. Famke Janssen/Jean Grey in X2) or it may be performed by a specialist voice actor. Voice-over is also commonly referred to as "off camera" commentary.
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Noun
1. the voice on an unseen commentator in a film of television program
(hypernym) voice, vocalization, vocalisation, phonation, vox

a commentary heard on a TV advertisement, but spoken by an off-screen announcer.