Fitness is the state of your body when you use it — how far you can run, how many flights of stairs you can climb without stopping, how fast your breathing settles afterwards. Fitness classes, fitness trackers, the fitness test on the first day of pre-season.
The older meaning is suitability: fitness for purpose is whether a thing does the job it was sold to do, and a court may ask about a defendant's fitness to stand trial or a parent's fitness to have custody. Biology has a third sense — an organism's fitness is how well it survives and passes on its genes, which is what survival of the fittest actually means: not the strongest, the best suited. Fitness is uncountable, so there is no plural and no a fitness. The adjective behind it is fit, which in British English means healthy and also, in casual speech, attractive.
1nounfitnessThe condition of being physically strong and healthy through regular exercise.
“Six weeks of cycling did more for his fitness than a year of good intentions.”
2nounsuitabilityThe quality of being suitable for a particular purpose or role.
“The report questioned his fitness to hold public office.”
synonyms: suitability
3nounbiologyIn biology, how successfully an organism survives and reproduces.
“Longer beaks gave the birds a measurable fitness advantage.”
fitness is a state or quality.
Fitness is fit with -ness on the end, and it appeared in the 1500s meaning suitability — whether a thing was fit for the job in hand. The gym meaning is young: the physical sense dates from 1935. So fitness for purpose is the senior sense, and the treadmill borrowed the word four centuries later.
Middle English
fit — an adversary of equal power
English
fit
English, today
fitness
From fit + -ness. 16th century, with the physical sense from 1935. fit Possibly from Middle English fit (“an adversary of equal power”), of unknown further origin. The original sense appears to have been "to marshal or deploy troops", which shifted to "to suit (troops to a certain location" > "suitable", and may be further related to Icelandic fitja (“to web, knit, cast on”).
Etymology from Wiktionary, CC BY-SA