(intransitive) To fail after a promising beginning; to disappoint expectations.

(idiomatic) A very short (but unspecified) amount of time; no time at all.

(optics) A disk of high-quality quartz glass having at least one side ground and polished with a deviation in flatness less than 50 nanometers all over, and a surface quality of 5 microfinish or less; used, in an interferometer, to measure the flatness of other surfaces.

Used other than figuratively or idiomatically: see flat, out. / Complete, total, downright. / Very busy. / (Australia) Lazy, sleeping.

Alternative spelling of mudflat.

(mathematics) Any Riemann space in which the metric tensors are constants throughout

(simile) Particularly or extremely flat.

(UK) a rate (typically of money) that does not vary over time

(chiefly American spelling) Alternative spelling of flat tyre

(rail transport) An at-grade junction between two crossing railroad tracks.