The action of natural selection consists in selecting individuals more adapted to a given ecological condition, eliminating those disadvantaged for that same condition. The most appropriate expression refers to the greater likelihood of a given individual surviving and leaving offspring in a given environment. Natural selection acts permanently on all populations. Even in stable and constant environments, natural selection acts in a stabilizing manner, is present, eliminating deviant phenotypes.
An adaptation is a feature that is common in a population because it provides some improved function. Adaptations are very fit for their functions and are produced by natural selection. Adaptations can take many forms: behavior that allows better predator avoidance, a protein that works best at body temperature, or an anatomical feature that allows the body to access valuable new resources - all of which can be adaptations. Many of the things that most impress us in nature are believed to be adaptations.