Who coined the term 'survival of the fittest'?

Herbert Spencer coined the term “survival of the fittest” to describe natural selection. He introduced the phrase in his 1864 book Principles of Biology, after reading Charles Darwin’s On the Origin of Species.

Spencer used the expression as a concise alternative to Darwin’s “natural selection.” Darwin later adopted it, beginning with the fifth edition of On the Origin of Species in 1869, while crediting Spencer for the wording. The two phrases are closely related, although Darwin’s explanation focused on how heritable variations that improve survival or reproduction become more common.

“Fittest” does not mean the strongest, fastest, or most athletic. In evolutionary biology, fitness means reproductive success: how effectively an organism survives and passes its genes to the next generation in a particular environment. A trait that is useful in one setting may be disadvantageous in another.

The phrase is also often confused with Social Darwinism, a later social and political interpretation of evolutionary ideas. Spencer himself applied evolutionary language to societies, but biological natural selection and claims about human social superiority are not interchangeable concepts.