When Charles Darwin was first coming up with the beginnings of the Theory of Evolution, he had to look for a mechanism that drove evolution. Many other scientists, such as Jean Baptiste Lamarck, had already described the change in species over time, but they did not offer an explanation as to how that occurred. Darwin and Alfred Russel Wallace independently came up with the idea of natural selection to fill that void in why species changed over time.
Natural selection is the idea that species that acquire adaptations that are favorable for their environment will pass down those adaptations to their offspring. Eventually, only individuals with those favorable adaptations will survive and that is how the species changes over time, or evolves through speciation.
In the 1800s, after Darwin first published his book On the Origin of Species, a British economist Herbert Spencer used the term "survival of the fittest" in relation to Darwin's idea of natural selection as he compared Darwin's theory to an economic principle in one of his books. This interpretation of natural selection caught on and Darwin himself even used the phrase in a later edition of On the Origin of Species. Clearly, Darwin used the term correctly as it was meant when discussing natural selection. However, nowadays this term is often misunderstood when used in place of natural selection.
A majority of the general public may be able to describe natural selection as "survival of the fittest". When pressed for a further explanation of that term, however, the majority will answer incorrectly. To a person not familiar with what natural selection really is, "fittest" means the best physical specimen of the species and only those in the best shape and best health will survive in nature.
This is not always the case. The individuals that survive are not always the strongest, fastest, or smartest. Therefore, "survival of the fittest" may not be the best way to describe what natural selection really is as it applies to evolution. Darwin did not mean it in these terms when he used it in his book after Herbert first published the phrase. Darwin meant "fittest" to mean the one best suited for the immediate environment. This is the basis of the idea of natural selection.
The individual of the population only needs to have the most favorable traits to survive in the environment. It should follow that individuals who have the favorable adaptations will live long enough to pass down those genes to their offspring. Individuals lacking the favorable traits, in other words the "unfit", will most likely not live long enough to pass down the unfavorable traits and eventually those traits will be bred out of the population. The unfavorable traits may take many generations to decline in numbers and even longer to disappear completely from the gene pool. This is evident in humans with the genes of fatal diseases are still in the gene pool even though they are unfavorable for the survival of the species.
Now that this idea is stuck in our lexicon, is there any way to help others understand the actual meaning of the phrase? Beyond explaining the intended definition of the word "fittest" and the context in which it was said, there is not really much that can be done. A suggested alternative would be to just avoid using the phrase all together when discussing the Theory of Evolution or natural selection.
It is completely acceptable to use the term "survival of the fittest" if the more scientific definition is understood. However, using the phrase casually without knowledge of natural selection or what it really means can be very misleading. Students, especially, who are learning about evolution and natural selection for the very first time should really avoid using the term until a deeper knowledge of the subject has been achieved.