Time domain

[1] It is used for the analysis of mathematical functions, physical signals or time series of economic or environmental data.

For discrete-time signals, the value is known at discrete, often equally-spaced, time intervals.

[3] An oscilloscope is a common tool used to visualize real-world signals in the time domain.

Though most precisely referring to time in physics, the term time domain may occasionally informally refer to position in space when dealing with spatial frequencies, as a substitute for the more precise term spatial domain.The use of the contrasting terms time domain and frequency domain developed in U.S. communication engineering in the late 1940s, with the terms appearing together without definition by 1950.

When analysis concerns the reciprocal units such as Hertz, then it is in the frequency domain.