Masking

In phonetics, the degree to which one part of a sound is masked by the rest of the sound is usually determined by two threshold measurements. The part of the sound that does the masking is called the masker component; the other is the masked component.

Comments
First, the intensity at which the masked component is just audible above the masker must be found: this is the masked threshold. Next, the intensity at which the masked component is just audible when sounded alone must be found: this is its absolute threshold. The ratio of these two intensities, expressed in decibels and called the threshold shift, is taken as a measure of masking.

However, masking can also take place when the tone and the mask are not presented at the same time. When the mask is presented after the tone, this is called backward masking. When the mask is presented preceding the tone, this is called forward masking.

Link
Utrecht Lexicon of Linguistics