multicyclic

Multicyclic is an adjective used in several scientific disciplines to describe systems that contain more than one cycle. In graph theory, a multicyclic graph is a connected graph that contains multiple cycles; equivalently its cyclomatic number μ = m − n + 1 is at least 2, where m is the number of edges and n the number of vertices. Such graphs have at least two independent cycles, meaning that removing a single edge will not destroy all cycles. Multicyclic graphs appear in network design, theoretical studies of cycle spaces, and other areas of graph theory.

In chemistry, multicyclic compounds are molecules that contain more than one ring. These include bicyclic and polycyclic compounds, where rings may be fused, spiro, or bridged. Multicyclic aromatic hydrocarbons, such as naphthalene and pyrene, feature fused ring systems and extended conjugation. The term is used to distinguish from monocyclic compounds that contain a single ring. Multicyclic structures often exhibit unique chemical reactivity, stability, and physical properties that are exploited in materials science, pharmaceuticals, and organic synthesis. The pattern of ring fusion or bridging affects aromaticity, electron distribution, and three-dimensional shape.

In practice, multicyclic describes a class rather than a single structure, and the specifics depend on the context. Related terms include bicyclic, polycyclic, and spiro compounds in chemistry, and concepts of cycle space and cyclomatic number in graph theory.