1. Expression of a variable as a function of time, as opposed to its expression as a function of frequency (frequency domain). Processing using time as the variable is done ‘‘in the time domain.’’ For example, convolving involves taking values at successive time intervals, multiplying by appropriate constants, and recombining; this is equivalent to filtering through frequency-selective circuitry. It is also equivalent to Fourier transforming, multiplying the amplitude spectra and adding the phase spectra (‘‘in the frequency domain’’), and then inverse-Fourier transforming. 2. Time-domain induced polarization is called the pulse method (q.v.). 3. For time-domain electromagnetic methods, see transient electromagnetic method.