Inchemistry, aring is an ambiguous term referring either to a simplecycle of atoms and bonds in a molecule or to a connected set of atoms and bonds in which every atom and bond is a member of a cycle (also called aring system). A ring system that is a simple cycle is called amonocycle orsimple ring, and one that is not a simple cycle is called apolycycle orpolycyclic ring system. A simple ring contains the same number ofsigma bonds as atoms, and a polycyclic ring system contains more sigma bonds than atoms.

A molecule containing one or more rings is called acyclic compound, and a molecule containing two or more rings (either in the same or different ring systems) is termed apolycyclic compound. A molecule containing no rings is called an acyclic oropen-chain compound.

A homocycle or homocyclic ring is a ring in which all atoms are of the samechemical element.^[1] Aheterocycle or heterocyclic ring is a ring containing atoms of at least two different elements, i.e. a non-homocyclic ring.^[2] Acarbocycle or carbocyclic ring is a homocyclic ring in which all of the atoms arecarbon.^[3] An important class of carbocycles arealicyclic rings,^[4] and an important subclass of these arecycloalkanes.

In common usage the terms "ring" and "ring system" are frequently interchanged, with the appropriate definition depending upon context. Typically a "ring" denotes a simple ring, unless otherwise qualified, as in terms like "polycyclic ring", "fused ring", "spiro ring" and "indole ring", where clearly a polycyclic ring system is intended. Likewise, a "ring system" typically denotes a polycyclic ring system, except in terms like "monocyclic ring system" or "pyridine ring system". To reduce ambiguity,IUPAC's recommendations onorganic nomenclature avoid the use of the term "ring" by using phrases such as "monocyclic parent" and "polycyclic ring system".^[5]

• Cyclic compound
• Polycyclic compound
• Heterocyclic compound
• Bicyclic molecule
• Spiro compound