Degradation. The alteration of material properties (e.g., strength, modulus, coefficient of expansion) which may result from deviations in manufacturing or from repeated loading and/or environmental exposure.

Delamination. Separation of the bond between the individual plies of a laminated composite structure.

Denominator. The lower part of a fraction (represented by the letter D in N⁄D), the quantity by which the numerator is divided.

Density. The weight of a substance per unit volume.

Detail drawing. A description of a single part, given in such a manner as to describe bylines, notes, and symbols the specifications for size, shape, material, and methods of manufacture to be used in making the part.

Detailed inspection. A thorough examination of an item including disassembly. The overhaul of a component is considered to be a detailed inspection.

Detonation. Uncontrolled burning of fuel in the cylinder of a reciprocating engine. Detonation causes explosive burning of the fuel which creates an increased cylinder pressure, excessive cylinder head temperature, and decreased engine performance.

Dew point. The temperature to which humid air must be cooled at constant pressure to become saturated.

Dial indicator. Measures variations in a surface by using an accurately machined probe mechanically linked to a circular hand whose movement indicates thousandths of an inch, or is displayed on a liquid crystal display (LCD) screen.

Diameter (circle). The length of a line passing directly through the center of a circle. Twice the radius of the circle.

Die. Used for cutting external threads on round stock.

Difference. The answer to a subtraction problem.

Direct current (DC). Electricity that flows in one direction at all times.

Directional stability. Stability about the vertical axis of an aircraft, whereby an aircraft tends to return, on its own, to flight aligned with the relative wind when disturbed from the equilibrium state.

Discontinuity. An interruption in the normal physical structure or configuration of a part, such as a crack, forging lap, seam, inclusion, porosity, and the like. A discontinuity may or may not affect the usefulness of a part.

Dissimilar metal corrosion. Caused by contact between dissimilar metal parts in the presence of a conductor.

Dividend. In a division problem, the number to be divided by the divisor. In 6 ÷ 2 = 3, the dividend is 6.

Division. The process of finding how many times one number (the divisor) is contained in another number (the dividend).

Divisor. In a division problem, the number by which dividend is to be divided. In 6 ÷ 2 = 3, the divisor is 2.

Doping. The process by which small amounts of additives called impurities are added to the semiconductor material to increase their current flow by adding a few electrons or a few holes.