The term natural selection refers to the the process where heritable traits that make it more likely for an organism to survive long enough to reproduce become more common over successive generations of a population. This can include anything from the exterior appearance of an organism (for example those that are most easily camouflaged tend to fair better) to the temperament of an organism (for example, a lion would have more difficulty in hunting prey if it was irrational and clumsy in behaviour instead of being stealthy and systematic in it's actions). Natural selection is a key mechanism of evolution.