Topic summary
Sigh

Extracted from the Wikipedia article Paralanguage.
Sighs
A sigh is a kind of paralinguistic respiration in the form of a deep and especially audible, single exhalation of air out of the mouth or nose, that humans use to communicate emotion. It is a voiced pharyngeal fricative, sometimes associated with a guttural glottal breath exuded in a low tone. It often arises from a negative emotion, such as dismay, dissatisfaction, boredom, or futility. A sigh can also arise from positive emotions such as relief, particularly in response to some negative situation ending or being avoided. Like a gasp, a yawn, or a moan, a sigh is often an automatic and unintentional act.