The site of the former World Trade Center towers in New York City destroyed on 11 September 2001.

(epidemiology) The initial patient in the population of an epidemiological investigation.

(of cattle) Kept in enclosures and never let out to graze.

plural of ground zero

(US, politics, historical) A 1980s plan whereby all Soviet and American nuclear weapons would be mutually withdrawn from Europe.

zero gravity

The point, during childbirth, at which the baby's head is level with the ischial spines in the maternal pelvis.

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

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

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