A bicyclic molecule is a molecule that features just two rings. Bicyclic molecules occur widely in organic and inorganic compounds. Among organic compounds, the two rings can be fully carbocyclic (all atoms in each ring all carbons), or one or both can be  heterocyclic (at least one atom in the rings not a carbon atom). Moreover, the two rings can both be aliphatic, or can be aromatic, or a combination of aliphatic and aromatic (cf. decalins and naphthalenes, norbornanes and  cyclophanes, aliphatic spiro compounds and  spiroaromatics, and biphenyl and cyclohexylbenzene, as aromatic and mixed examples).