voice-over
US /ˈvɔɪs.oʊ.vər/
UK /ˈvɔɪs.oʊ.vər/

1.
a piece of narration in a movie or broadcast, not accompanied by an image of the speaker.
:
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The documentary used a powerful voice-over to tell the story.
•
She recorded the voice-over for the commercial.