Fitness
See this discussion of the meaning of fitness in biology compared to everyday usage, published by biologist Sedeer el-Showk in Nature’s Scitable blogs. Quote:
Thanks to the phrase “survival of the fittest”, fitness is quite a prominent idea in the popular perception of evolution. The description was originally coined by Herbert Spencer after he read On the Origin of Species; Darwin adopted it in later editions and it’s been popular ever since. Unfortunately, this glib phrase often eclipses a more accurate depiction of evolution, leading to some common misunderstandings. This confusion arises because “fitness” has a different meaning in evolutionary biology than it does in general usage.
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Organisms (or genes or replicators) don’t survive because they are fit; rather, they are considered fit because they survived.
See also Natural Selection.