Functional group is a term frequently used in organic chemistry to describe specific groups of atoms that are responsible for the characteristic chemical reactions and properties of a molecule. Synonyms for functional group include chemical moiety, reactive group, chemical group, and substituent. Chemical moiety refers to a specific part of a molecule, while reactive group emphasizes the reactivity of the functional group. The term substituent is used when the functional group replaces a hydrogen atom in a molecule. Other synonyms include ligand, residue, and radical, which have more specialized and specific meanings in certain contexts. Regardless of the chosen synonym, understanding the functions and properties of these groups is crucial for understanding the behavior of organic molecules.