In electronics and derivative fields such as telecommunications, a delay line is rigorously defined as a single-input-channel device, in which the output channel state at a given instant, t, is the same as the input channel state at the instant t-n, where n is a number of time units, i.e., the input sequence undergoes a delay of n time units, such as n femtoseconds, nanoseconds, or microseconds. (The delay line may have additional taps yielding output channels with values less than n.) In other words, the device introduces a delay of a certain amount of time between its input and output. Specific devices and approaches:

• A transmission line or equivalent device (such as an analog delay line) used to delay a signal.
• In computer simulation it can be used as a numerical technique, also called a bi-directional delay line or transmission line modelling (TLM) method.
• In digital logic and digital signal processing, a delay line refers to a sequential logic element in which the output state at a given time n (in samples) is the same as the input was at the time n-L. Delay lines are generally expressed as z^-L, where L is the length of the delay in samples. A unit delay (a delay of one sample) is therefore expressed as z^-1.
• In computing, delay line often refers to delay line memory.