Smell, scent, odor, aroma all denote a property of a thing that makes it perceptible to the olfactory sense.
Smell not only is the most general of these terms but tends to be the most colorless. It is the appropriate word when merely the sensation is indicated and no hint of its source, quality, or character is necessary.
It is also the preferred term when accompanied by explicitly qualifying words or phrases and occasionally, even when unqualified, it implies offensiveness.
Scent tends to call attention to the physical basis of the sense of smell and is particularly appropriate when the emphasis is on the emanations or exhalations from an external object which reach the olfactory receptors rather than on the impression produced in the olfactory centers of the brain, but scent can apply specifically to emanations evidencing the passing of a body (as an animal) and may suggest a high level of sensory efficiency in a perceiver or from its use as a synonym of perfume the term may suggest a pleasant quality.
Odor is oftentimes in distinguishable from scent , for it too can be thought of a something diffused and as something by means of which external objects are identified by the sense of smell. But the words are not always interchangeable, for odor usually implies abundance of effluvia and therefore does not suggest, as scent often does, the need of a delicate or highly sensitive sense of smell.
For these reasons odor usually implies general perceptibility and is the normal word in scientific use especially when the classification or description of types is attempted.
Aroma usually adds to odor the implication of a penetrating, pervasive, or, sometimes, a pungent quality; it need not imply delicacy or fragrance, but it seldom connotes unpleasantness, and it often suggests something to be savored, with the result that it is used of things that appeal both to the sense of smell and taste or by extension to one’s aesthetic sense.