Define the term "substituent" in organic nomenclature.

The term "substituent" specifically refers to an atom or group of atoms that replaces one or more hydrogen atoms in a hydrocarbon structure. Substituents can be various functional groups, such as alkyl groups, halogens, or other functional groups, and they modify the properties and reactivity of the parent hydrocarbon. For example, in toluene (methylbenzene), the methyl group (-CH₃) is considered a substituent that replaces one of the hydrogen atoms in benzene.

In organic nomenclature, identifying substituents is essential for naming compounds correctly, as the presence and positioning of these groups influence both the name and the structural formula of the organic molecule. This distinction helps chemists predict the behavior and characteristics of the molecules during chemical reactions and interactions.