Tumor heterogeneity refers to the genetic, epigenetic, and phenotypic variations that exist within a single tumor or among different tumors in the same individual. This heterogeneity can arise due to factors such as mutations, gene expression differences, and cellular interactions within the tumor microenvironment. Understanding tumor heterogeneity is important for developing targeted treatment strategies and predicting treatment response, as different subpopulations of cells within a tumor may respond differently to therapy. Researchers use various techniques, such as single-cell sequencing and imaging, to study tumor heterogeneity and its impact on cancer progression and treatment outcomes.