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"Fitness" can mean a number of different things, depending on the specific context in which it's found. In the context of evolutionary theory, 'fitness' is a shorthand reference to 'reproductive fitness', i.e. the ability to produce fertile and viable offspring, or 'environmental fitness' meaning that an organism is able to live and reproduce in a particular environment.
The word is often used in an equivocal sense, by applying some other notion of fitness, and then arguing that therefore Darwinian fitness is untrue, or converse, arguing from reproductive fitness that the object under discussion is "better" in some other fashion.
In evolutionary theory, fitness is observationally defined: an organism supplied with its needs will reproduce, and is therefore "reproductively fit", organisms that commonly reproduce and maintain populations in a particular environment are "fit". Therefore there is the trap of using "fitness" tautologically: by arguing from the observation of a population's presence, to, for example, imply that there is no better organism for that niche in that environment. Because environmental fitness doesn't imply that the organism is the best possible organism, merely that it is good enough to reproduce in that particular environment, and that no other organism has yet been selected in favor of in that environment.
It is often better to think of the word as fit-ness, as in the quality of "fitting" into an environment as opposed to "fitness" as in "physical fitness".