An isomer is one of two or more compounds that share the same molecular formula but differ in the arrangement of their atoms. Isomers matter because such compounds can have markedly different physical and chemical properties despite identical atom counts. For example, two compounds may both have the formula C₄H₁₀ yet differ as a straight chain and a branched chain; the general phenomenon is called isomerism.

异构体是分子式相同但原子排列方式不同的两种或多种化合物之一。异构体之所以重要,是因为这类化合物尽管原子数目完全相同,却可能具有显著不同的物理和化学性质。例如,两种化合物的分子式都可以是 C₄H₁₀,却分别为直链和支链;这一普遍现象称为同分异构。