An adaptation is a heritable feature that has become common because it increased reproductive success in a particular environment over generations.

Adaptation is therefore historical and population-level. An individual does not usually acquire a new genetic adaptation because conditions become difficult.

This differs from acclimation, a reversible physiological or behavioral adjustment within an individual's lifetime. For example, increasing red-blood-cell production after moving to high altitude is acclimation. Genetic variants that improve oxygen transport and have risen in frequency over many generations can represent adaptation.

Adaptations are also environment-dependent. Thick fur can increase fitness in a cold climate but impose a heat cost in a warm climate. There is no universally 'best' phenotype independent of ecological context.

An adaptation need not be perfect. Evolution works with inherited variation, developmental constraints and trade-offs. A structure may perform several functions, and selection can modify existing structures rather than designing from scratch.

The causal direction is important:

heritable variation
        ↓
differences in reproductive success in an environment
        ↓
changes in variant frequencies over generations
        ↓
population becomes better fitted to that environment

It is therefore misleading to say that organisms evolve a feature because they need it. Variation arises first; selection changes the frequencies of variants according to their consequences for fitness.

Adaptation is the resulting match between inherited traits and an environment produced by past natural selection.