A feature on some television sets that allows the monitoring of two channels simultaneously, with a smaller picture inset in the corner of the main picture; a picture of this type.
Origin
1980s; earliest use found in Fortune.
Definition of picture-in-picture in US English:
picture-in-picture
noun
A feature on some television sets that allows the monitoring of two channels simultaneously, with a smaller picture inset in the corner of the main picture; a picture of this type.