The primary characteristic of an organism is its ability to grow, reproduce, and respond to stimuli within its environment. Organisms, whether unicellular or multicellular, share fundamental processes such as metabolism and homeostasis that govern their survival. This characteristic distinguishes living entities from non-living matter. Furthermore, organisms exhibit genetic variation, allowing them to adapt to their surroundings and evolve over generations. Understanding these traits is crucial for studying biology and the complexities of life on Earth.