D (in RMA) - The Deadline for a service which is relative to a request for the service.
DAC - Digital to Analog Conversion, most often used to provide analog output to an actuator from a digital I/O interface - e.g. a motor or speaker. Data Flow Diagram - Data Flow Diagram, a diagram used in Structured Analysis/Design which indicates where data in the system originates, where terminate and how it is processed in between. (Note that a DFD typically includes a CFD and therefore shows both data flow and control flow).
Data Segment - a memory region reserved for global variables and constants in a C program thread - most often each thread has its own data segment. (Note, most programs include a Stack, Data, and Text segment as a minimum).
Datagram Transport - transmission of packets on a link such that errors in transmission can be detected, but are not automatically corrected nor is there automatic retransmission of lost data - furthermore, there is no concept of a connection (real or virtual) such that multiple messages are unrelated and if fragmented will not be reordered or reassembled automatically.