Real Dictionary

sniff
verb: When you sniff, you breathe in air through your nose hard enough to make a sound, for example when you are trying not to cry, or in order to show disapproval.

sniff in American English
to breathe in forcibly through the nose; draw in or inhale nasally
to draw in air through the nose with enough force to be heard, as in clearing the nose or smelling something
intransitive verb: to draw air through the nose in short, audible inhalations
noun: an act of sniffing; a single, short, audible inhalation
transitive verb: to perceive by or as by smelling

sniff in British English
noun: the act or sound of sniffing
verb: to inhale through the nose, usually in short rapid audible inspirations, as for the purpose of identifying a scent, for clearing a congested nasal passage, or for taking a drug or intoxicating fumes