What is the X in X-mas?
olfaction ol·fac·tion (ŏl-fāk'shən, ōl-)
The sense of smell.
the detection and identification by sensory organs of airborne chemicals. The concept of smell, as it applies to humans, becomes less distinct when invertebrates and lower vertebrates (fish and amphibians) are considered, because many lower animals detect chemicals in the environment by means of receptors in various locations on the body, and no invertebrate possesses a chemoreceptive structure resembling the vertebrate nasal cavity. For this reason, many authorities prefer to regard smell as distance chemoreception and taste as contact chemoreception.