Smell receptors, also known as olfactory sensors, were a category of sensors that gave droids the capability of artificial olfaction. The 3PO-series protocol droids from Cybot Galactica utilized these sensors, as did the KX-series security droids manufactured by Arakyd Industries.
The reference book Star Wars: Droid Factory was the first source to mention olfactory sensors for droids. Nevertheless, Return of the Jedi: Beware the Power of the Dark Side! states that droids are incapable of smelling, but can instead analyze the chemical makeup of the surrounding atmosphere through their chem-receptors.