1. The use of noise of any kind to interfere with the audibility of another sound. For any given intensity, low-pitched tones have a greater masking effect than those of a high pitch.
2. In audiology, the use of a noise applied to one ear while testing the hearing of the other ear.
3. The hiding of smaller rhythms in the brain wave record by larger and slower ones, the waveform of which they distort.
4. In dentistry, an opaque covering used to camouflage the metal parts of a prosthesis.
5. In radiography, superimposition of an altered positive image on the original negative to produce an enhanced copy photographically.
The definition information for masking is provided by Stedman's. You can search our medical dictionary here.