(1):

(n.) The act of tapping or striking the surface of the body in order to learn the condition of the parts beneath by the sound emitted or the sensation imparted to the fingers. Percussion is said to be immediate if the blow is directly upon the body; if some interventing substance, as a pleximeter, is, used, it is called mediate.

(2):

(n.) Hence: The effect of violent collision; vibratory shock; impression of sound on the ear.

(3):

(n.) The act of percussing, or striking one body against another; forcible collision, esp. such as gives a sound or report.