Grue and bleen are artificial predicates, coined as two portmanteaux of "green" and "blue" by philosopher Nelson Goodman in his book Fact, Fiction, and Forecast. The words are used to illustrate what Goodman calls "the new riddle of induction".

Goodman defined grue relative to an arbitrary but fixed time t as follows: "The predicate grue ... applies to all things examined before t just in case they are green but to other things just in case they are blue." The word bleen has a complementary definition: An object is bleen if it is blue and examined before time t, or else green.

To understand the problem Goodman posed, it is helpful to imagine some particular future time t, say January 1, 2022.

Some popularizations of the concept have described it in a slightly different way: an object is grue if it is green when examined before time t and blue when examined afterwards (and likewise for bleen). That version is different because it envisions the same object as appearing green or blue at different times, while the original definition means that the object is always green or always blue, but which one depends on when the object was first observed. This article will deal only with the original definition.