Strobing is an effect that occurs when narrow stripes appear on video, in which the stripes flicker. The main culprit is neckties, as they tend to have small stripes at a diagonal angle, but striped shirts or other striped objects can also cause it (for instance, tennis matches can have flickering white lines). Strobing mainly occurs on live TV with guests; experienced TV people tend to avoid clothing that will cause it.
As this is essentially a visual artifact of Ludicrous Precision, higher definition video, such as Digital TV and HDTV, actually tends to make it worse. Counterintuitively, video can sometimes actually improve in quality if it is slightly blurred.
In analogue systems the shimmering effect is caused by the colour being carried over the same signal as the luminance using a subcarrier. Stripes cause a high frequency luminance pattern when the television mistakes for the colour subcarrier.
Closely related to the concept of Moire patterns. Also compare Raster Vision.