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The term 'delay line' has multiple meanings:

★ In electronics and derivative fields such as telecommunications, a delay line is a device where the input signal reaches the output of the device after a known period of time has elapsed. It 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''.) Specific devices and approaches:

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★ A transmission line or equivalent device (such as an analog delay line) used to delay a signal.

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★ In computer simulation it can be used as a numerical technique, also called a bi-directional delay line or transmission line modelling (TLM) method.

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★ In digital logic and digital signal processing, a ''delay line'' or digital 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''.

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★ In computing, ''delay line'' often refers to delay line memory.

★ In Optics : cavity delay lines or trombone delay lines e.g. [1]

★ In sound reenforcement, the 'delay line' refers to the system of loudspeakers used to supplement the main delivery system in areas where it does not reach effectively, such as the back of a large (deep) room, or underneath a balcony. The speakers are sent a signal that has been delayed in relation to the mains.

★ In neurobiology, 'delay lines' can refer to neurons, the cells that transmit electrical information in the brain. Electrical conduction in neurons is not instantaneous; it is delayed depending on the length of the axon and other properties of the neuron (e.g. myelination). This delay can be used for time sensitive calculations; the canonical example is the calculation of inter-aural time differences in the pons, used for sound localization.
''An earlier version of this page came from Federal Standard 1037C and from MIL-STD-188.''