Isomerism is a phenomenon in chemistry where two or more compounds have the same molecular formula but different structural arrangements. This means that while they contain the same number and types of atoms, their properties can vary significantly due to differences in how those atoms are connected or arranged in space. There are two main types of isomerism: structural isomerism, where the connectivity of atoms differs, and stereoisomerism, where the spatial arrangement of atoms is different. Isomers can exhibit distinct physical and chemical properties, making isomerism an important concept in fields like organic chemistry and pharmaceuticals.