Crosstalk measurement is made on audio systems to determine the amount of signal leaking across from one channel to another.
Interchannel crosstalk between the two channels of a stereo system, and is usually not very important on modern systems, though it was hard to keep below the desired figure of -30dB or so on vinyl recordings and FM radio.
A relatively high level of crosstalk between stereo channels is normally of no consequence, since most sound sources will have been mixed to both channels to some extent anyway, and a small extra amount due to crosstalk will not normally affect the stereo image. It is important though that any significant crosstalk is not in the form of distortion, something that is often missed in relation to power amplifiers. In a power amplifier the currents flowing in power rails can be essentially half-wave rectified signals if they original from one side of a push-pull output circuit. If these waveforms couple into the other channel they can contribute serious distortion. Though not a common problem, this is a possibility that should be borne in mind.
Crosstalk between channels in mixing consoles, and between studio feeds is much more of a problem, as these are likely to be carrying very different programmes or material which is likely to be heard in quiet passages unless it is at a very low level.
The IBA drew up a weighting curve for use in crosstalk measurement that gives due emphasis to the subjective audibility of different frequencies, as shown here. This is still in use, despite the demise of the IBA, and in the absence of any international standards is worth adopting.