A Heterocyclic Compound is a class of organic compounds with cyclic structure, also called heterocycle that has atoms of at least two different elements as members of its ring, in addition to carbon, also include one or more nitrogen, sulphur, oxygen and other atoms. Those containing one ring are called single-heterocyclic compounds, such as furan, thiazole, and pyridine. Those containing two or more rings are called fused heterocyclic compounds such as quinoline and purine. The most common and most stable are five- or six-membered heterocyclic ring. (Refer to the Ring Structure)

Reference Definition by Chemistry.msu.edu: A Heterocyclic Compound is the compounds classified as heterocyclic probably constitute the largest and most varied family of organic compounds. After all, every carbocyclic compound, regardless of structure and functionality, may in principle be converted into a collection of heterocyclic analogs by replacing one or more of the ring carbon atoms with a different element. Even if we restrict our consideration to oxygen, nitrogen and sulphur (the most common heterocyclic elements), the permutations and combinations of such a replacement are numerous.