polycycle

Polycycle is a term used in chemistry and graph theory to denote a structure composed of multiple cycles, typically rings in organic compounds or cycles in graphs. In chemistry, polycycle refers to molecules that contain two or more rings in a single framework. The rings may be fused (sharing one or more edges) or linked by single bonds. Polycyclic aromatic hydrocarbons (PAHs) are the best-known class, consisting of fused aromatic rings arranged in planar, conjugated systems. Examples include naphthalene (two fused rings), anthracene and phenanthrene (three fused rings in different arrangements), pyrene, and coronene. The pattern of ring fusion influences a compound’s stability, electronic structure, and reactivity. Some polycycles are non-aromatic or non-planar, and several occur in natural products, pharmaceuticals, and materials science.

In graph theory, polycycle is used to describe a connected graph built from repeating cycles. The precise definition varies by author, but such graphs are often characterized by their decomposition into cycles that intersect at vertices or along edges. In chemical graph theory, polycycle graphs help model properties related to connectivity and resonance and provide abstract representations of polycyclic compounds.

Related terms include polycyclic (as an adjective form), polycyclic aromatic hydrocarbons, fused ring systems, and multilayer ring architectures.