(1):

(n.) The act of retarding; hindrance; the act of delaying; as, the retardation of the motion of a ship; - opposed to acceleration.

(2):

(n.) The extent to which anything is retarded; the amount of retarding or delay.

(3):

(n.) The keeping back of an approaching consonant chord by prolonging one or more tones of a previous chord into the intermediate chord which follows; - differing from suspension by resolving upwards instead of downwards.

(4):

(n.) That which retards; an obstacle; an obstruction.