O meaning in Chemistry ?

An odor (American English) or odour (British English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive by their sense of smell. An odor is also called a "smell" or a "scent", which can refer to either a pleasant or an unpleasant odor.

While "odor" can refer to pleasant and unpleasant odors, the terms "scent", "aroma", and "fragrance" are usually reserved for pleasant-smelling odors and are frequently used in the food and cosmetic industry to describe floral scents or to refer to perfumes.

In the United Kingdom and other Commonwealth, English-speaking nations, "odour" refers to scents in general—without positive or negative connotations; but in the United States, and for many non-native English speakers around the world, "odor" generally has a negative connotation as a synonym for "stink". An unpleasant odor can also be described as "reeking" or "smelly", and can also be called a "malodor" or "stench".