The term for a genetic change that can affect an organism's fitness is a mutation. Mutations are alterations in the DNA sequence that can be caused by various factors, including environmental influences or replication errors. Depending on their nature, mutations can be beneficial, harmful, or neutral, influencing an organism's ability to survive and reproduce in its environment. Understanding mutations is crucial in fields such as evolutionary biology, genetics, and medicine, as they contribute to variability within populations and can lead to adaptations.