Definition of fitness

noun: fitness

1.

the condition of being physically fit and healthy.

"disease and lack of fitness are closely related"

synonyms:health, strength, robustness,sturdiness, hardiness, vigour,lustiness, stalwartness; 

athleticism,toughness, ruggedness, physical fitness, muscularity, tone; 

good health, good condition, good shape,well-being; 

state of health, condition,shape, trim, fettle;

BIOLOGY

an organism's ability to survive and reproduce in a particular environment.

plural noun: fitnesses

"if sharp teeth increase fitness, then genes causing teeth to be sharp will increase in frequency"

2.

the quality of being suitable to fulfil a particular role or task.

"the medical board assessed his fitness for active service"

"he was called before medical boards to assess his fitness for active service"