direction finder. See DF.
dirigible. A large, steerable, cigar-shaped, self-propelled lighter-than-air craft. Dirigibles are made of a rigid truss structure covered with fabric. Gas bags inside the structure contain the lifting gas, which may be either hydrogen or helium.
discontinuity. In nondestructive inspection, any interruption in the normal physical structure or configuration of a part. A discontinuity may or may not affect the usefulness of a part.
discreet. Having good judgment. A discreet action is one that gives the desired results without being showy or calling attention to itself.
discrete. Separate, not continuous. A discrete change is made in a number of separate steps, rather than in a continuous, smooth movement.
discrete code (air traffic control). As used in the air traffic control radar beacon system (ATCRBS), a discrete code is any one of the 4096 selectable Mode 3/A aircraft transponder codes except those ending in zero zero. Examples of discrete codes are 0010, 1201, 2317, and 7777.
Nondiscrete codes, such as 0100, 1200, and 7700, are normally reserved for radar facilities that are not equipped with discrete decoding capability and for other purposes such as emergencies (7700) and VFR aircraft (1200).