Mutation Stop
Any organ-stop not tuned to the diapason or any of its octaves, as the tierce, quint, twelfth, larigot, etc. Stops of this kind (also mixtures, cornets, sesquialteras) are used for the purpose of "filling up" the volume of tone and giving it greater brilliancy.
Mute [It., sordino; Fr., sourdine; Ger., Dämpfer]. A small contrivance of wood or metal placed on the bridge of the violin, etc., to deaden the sound; a cone or cylinder of pasteboard, leather, or wood placed in the bell of a brass instrument for the same purpose.