It's technically known as ellipsis, and even more technically known as exophoric ellipsis.

Exephoric ellipsis is the omission of words that can be recovered (i.e. worked out) from the non-linguistic situation, and is most often used in informal conversation, e.g. Coming! for I'm coming .

Cataphoric ellipsis is the omission of words that can be recovered from the grammatical context, e.g. We phoned the plumber and asked him to come tomorrow, where the repeated subject (we asked) has been omitted.

Bingley

Bingley