(physics) A spectrum produced by a diffraction grating whose adjacent slits are one wavelength apart.

(programming) An exception (error condition) when handled by a debugger, such that the programmer has the first chance to study it; such an exception would otherwise proceed to a handler or (in its absence) crash the program.

plural of first-chance exception

(statistics) type I error

plural of error of the first kind

(derogatory) A country or region regarded as under the control of the United States.

plural of sergeant first class

plural of first-aid box

plural of first-time buyer

plural of first-person singular