Inmathematics andsignal processing, thetime domain is a representation of how a signal, function, or data set varies with time.[1] It is used for the analysis ofmathematical functions, physicalsignals ortime series ofeconomic orenvironmental data.
In the time domain, theindependent variable is time, and thedependent variable is the value of the signal. This contrasts with thefrequency domain, where the signal is represented by its constituent frequencies. Forcontinuous-time signals, the value of the signal is defined for allreal numbers representing time. Fordiscrete-time signals, the value is known at discrete, often equally-spaced, time intervals.[2] It is commonly visualized using a graph where the x-axis represents time and the y-axis represents the signal's value.[3] Anoscilloscope is a common tool used to visualize real-world signals in the time domain.
Though most precisely referring totime in physics, the termtime domain may occasionally informally refer toposition inspace when dealing withspatial frequencies, as a substitute for the more precise termspatial domain.
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The use of the contrasting termstime domain andfrequency domain developed in U.S.communication engineering in the late 1940s, with the terms appearing together without definition by 1950.[4] When an analysis uses thesecond or one of its multiples as aunit of measurement, then it is in the time domain. When analysis concerns the reciprocal units such asHertz, then it is in the frequency domain.
