ellipsis (grammar)
Definition - Omitting a word or phrase that is needed for complete syntax, but which isn't needed for complete comprehension.

Example -
In the sentence Fire when ready the you are is missing but inferred, thus the sentence really means Fire when you are ready.

Etymology -
The word derives from the Greek elleipsis, a leaving out (from Greek elleipein, to leave out).

Oxford English Dictionary -
The first recorded use of the word in its grammatical sense is from 1612: "The first of the Substantives is oft understood by a figure called Ellipsis."
(Brinsley Pos. Parts (1669) 67)

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