The start of the first hour of the day on the 24-hour clock, i.e. 0:00, military time.

(military) The location of the center of a burst of a nuclear weapon at the instant of detonation. The zero point may be in the air, or on or beneath the surface of land or water, depending upon the type of burst, and it is thus to be distinguished from ground zero.

The state of apparent weightlessness which occurs in a very low gravity field, or in free fall.

(logic) Describing situations in which the veracity of a statement may be shown to be true without revealing any other information.

(US) The smallest size of women's clothing / (informal) A woman with a very low body mass index, especially such a fashion model

(education) A high school class that is offered early in the morning, before the scheduled start of the school day.

(grammar) The absence of an ending after a noun‐stem in a language in which endings are normally applied; often written as ∅

(mathematics) A type of potential counterexample to the generalized Riemann hypothesis, on the zeros of Dirichlet L-function.

(mathematics) A matrix whose entries are all zero.

(category theory) An object which is both an initial object and a terminal object.