grue

Grue is a fictional monster from the text adventure game Zork, lurking in dark places and devouring players who venture without a light source: It is pitch black. You are likely to be eaten by a grue.

The term has become gaming shorthand for unseen dangers in darkness.

Separately, in philosophy, grue is a constructed predicate—"green if observed before time T, blue otherwise"—used in discussions of induction and the problem of defining natural properties.