Damping: (1) The prevention of free swinging or vibration by some means, usually friction or resistance. (2) The dissipation of energy with motion or time.

Decay Time: The time required for the trailing edge of a pulse to decrease from 90 percent to 10 percent of its maximum amplitude.

Defect: Nonfulfillment of an intended usage requirement of reasonable expectation, including one concerned with safety.

Degree of Documentation: Extent to which evidence is produced to provide confidence that specified requirements are fulfilled.

Demineralization: Removal of mineral constituents from water.

Deionization: Removal of ionized minerals and salts from a solution by a two phase ion exchange procedure.

Density: The mass per unit volume. CGS unit: gm/cm

DI Water: Deionized water.

Dial Indicator: This is a mechanical lever system used for amplifying small displacements and measuring it be means of a pointer which transverses a graduated dial.

Dialysate Meter: Verifies the total concentration of ionized salts in dialysate solutions used in hemodialysis or kidney equipment.

Differential Voltmeter: A voltmeter that operates on the potentiometric principle. The unknown voltage is compared to an adjustable calibrated voltage developed within the differential voltmeter.

Differentiating Circuit: A circuit in which the output voltage is proportional to the rate of change of the input voltage.

Diffraction: When light passes sharp edges or goes through narrow slits the rays are deflected and produce fringes of light and dark bands.

Digital Voltmeter: An electronic voltmeter that gives readings in digits.

Diopter: A unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters.

Direct Current (DC): A current with a constant polarity.

Disposition of nonconformity: Action to be taken to deal with an existing nonconforming entity in order to resolve the nonconformity.

Distortion: Any deviation from the desired waveform.

Double-Pole, Double-Throw (DPDT): A term used to describe a switch or relay output contact form. Two separate switches that operate simultaneously each with a normally open and normally closed contact and a common connector.

Drift: Slow change of a metrological characteristic of a measuring instrument.

Dyne: The unit of force which, when acting upon a mass of 1 gm, will produce an acceleration of 1 cm/sec/sec.