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A hot pixel is a CCD (or similar sensor) pixel that shows brighter than it should, i.e., produces a higher count for the position it is imaging than specified for the sensor's pixels. Such a hot pixel might be especially efficient (out of line with the CCD's specifications) or might be otherwise flawed, adding noise. The analogous terms, warm pixel and cold pixel are also used pixels showing fewer extraneous counts or too-low counts.
Dithering is slightly changing a telescope's aim so that each CCD pixel images a slightly different part of the sky than before. This is used as a work-around: if a bright spot in the raw image remains with the same pixel rather than moving to the image pixel associated with the same portion of the sky, then the problem is evident, and through repeated dithering, the likely culprit pixel can be determined and processing can be carried out to effectively ignore it.