突变（法语，德语，英语：Mutation，即基因突变）在生物学上的含义，是指细胞中的遗传基因（通常指存在于细胞核中的脱氧核糖核酸）发生的改变。它包括单个碱基改变所引起的点突变，或多个碱基的缺失、重复和插入。原因可以是细胞分裂时遗传基因的复制发生错误、或受化学物质、基因毒性、辐射或病毒的影响。

突变通常会导致细胞运作不正常或死亡，甚至可以在较高等生物中引发癌症。但同时，突变也被视为演化的“推动力”：不理想的突变会经天择过程被淘汰，而对物种有利的突变则会被累积下去。中性突变（neutral mutation）对物种没有影响而逐渐累积，会导致间断平衡。

In biology, a mutation is a permanent alteration of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA or other genetic elements. Mutations result from damage to DNA which is not repaired, errors in the process of replication, or from the insertion or deletion of segments of DNA by mobile genetic elements. Mutations may or may not produce discernible changes in the observable characteristics (phenotype) of an organism. Mutations play a part in both normal and abnormal biological processes including: evolution, cancer, and the development of the immune system, including junctional diversity.