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Grue is an artificial predicate, coined from "green" and "blue" by philosopher Nelson Goodman in one of the seminal works in the philosophy of science, Fact, Fiction, and Forecast. The predicate is used to illustrate what Goodman calls the new problem of induction.
The predicate is defined relative to an arbitrary time t as follows: An object satisfies the predicate "x is grue" if and only if it is first observed before t and is then found to be green, or is first observed after t and is then found to be blue.
The problem is this. Let t be some moment in the future. Then "All emeralds are green" and "All emeralds are grue" are both true. So long as t has not yet arrived, every green emerald we find agrees with both sentences, but surely a green emerald is evidence only for "All emeralds are green", not evidence for "All emeralds are grue." The problem is to explain why not.
Casually, "grue" is used to mean "green before January 1, 2000 and blue on or after January 1, 2000"; sometimes this is useful for metaphysical or epistemological discussions. It should be noted that the above definition does not require that any objects change colour; grue is, strictly speaking, not a colour but a complex property that is a function of both colour and time of first observation.
"Grue" has also been used as a blanket term to translate colour words in some languages. A large number of the world's languages, including Welsh and Ubykh, do not distinguish colour terms for "green" and "blue", using the same word for both. The word "grue" is occasionally used to translate these colour terms.
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