Glossary:

Captions, subtitles 
Lines of printed text on the screen that are direct representations of spoken or signed text in a programme.
ChromaKey 
A method of representing two separate video signals simultaneously on a screen, with complete, partial, or no overlap between the two signals. When used for sign language videos, it is as if the signer stands in front of a movie-screen on which the other video signal is projected.
Gloss
A gloss is a printed word used as a graphic representation of a sign. Glosses are not exact translations of signs. A text printed in glosses is not a complete representation of that text in sign language. Glosses are usually written in capital letters. 
Picture-in-picture, sign language bubble, box
A method of representing two separate video signals simultaneously on a screen, with no overlap between the two signals. The second signal is shown in a box somewhere on the screen. The box can have any shape (bubble, square, oval) and size, and can be placed anywhere on the screen. The edges of the box can be clearly demarcated or can blend in with the background.
Voice-over
Spoken language that is recorded later and added to a production, e.g. a translation of sign language texts that are presented on the screen.