Origin: Germanic Old English suffix
Fitness has 4 different meanings across 1 category:
the quality of being suitable
"they had to prove their fitness for the position"
good physical condition; being in shape or in condition
"After three months of daily running, she finally got her fitness back to pre-pregnancy levels."
the quality of being qualified
"With his years of experience and specialized certifications, she has the fitness to lead the new safety initiative."
The condition of being fit, suitable or appropriate.
"The software's fitness for running large-scale simulations was confirmed after passing all stress tests."
In plain English: Fitness is how healthy and strong your body is.
"Her daily jogging routine keeps her in excellent fitness."
Usage: Use fitness to describe how well someone's physical body functions or how suitable something is for a specific purpose. Avoid confusing it with financial wealth or general healthiness unless referring specifically to physical capability.
The word fitness is formed by adding the suffix -ness to the adjective fit and first appeared in English during the 16th century. While it originally described general suitability or condition, its specific meaning referring to physical health did not emerge until 1935.