Dictionary.net

A short breathing. A slow, voluntary contraction of the diaphragm and intercostal muscles, the effect of which is to restore the due relation between the circulation and respiration, or to free ourselves from an inconvenient sense of weight felt in the chest, when we are much depressed, - a weight, which appears to be chiefly dependent on the functions of the heart being disturbed through the moral influence. The sigh differs from the sob :-the latter being involuntary and spasmodic. Sighing, occurring in fevers to a great extent, indicates considerable depression. When accompanied with a vocal sound, it constitutes a groan.