In evolutionary biology, fitness measures reproductive contribution to future generations. It is not simply strength, health or lifespan.

An organism can survive for a long time yet leave no descendants, giving it low evolutionary fitness. Another may live a shorter life but leave many surviving offspring, giving it greater fitness.

Fitness is usually relative: what matters is reproductive success compared with other individuals or genotypes in the same population and environment.

If genotype $AA$ leaves on average 8 surviving offspring, $Aa$ leaves 6 and $aa$ leaves 4, we can scale the largest value to 1:

$$w_{AA}=1,\qquad w_{Aa}=\frac{6}{8}=0.75,\qquad w_{aa}=\frac{4}{8}=0.50.$$

These $w$ values are relative fitnesses. The corresponding selection coefficient is often written

$$s=1-w.$$

Thus $aa$ has $s=0.50$ relative to the reference genotype in this example.

Fitness depends on environment. An allele that increases reproductive success in one habitat can reduce it in another. Fitness can also involve several life-history components: surviving to reproductive age, obtaining mates, producing offspring and helping offspring survive.

The key idea is

$$\boxed{\text{fitness} = \text{relative genetic contribution to later generations}.}$$

Natural selection changes populations when heritable differences are associated with differences in fitness.