Teleology (from τέλος, telos, 'end,' 'aim,' or 'goal,' and λόγος, logos, 'explanation' or 'reason') or finality is a reason or an explanation for something which serves as a function of its end, its purpose, or its goal, as opposed to something which serves as a function of its cause.

A purpose that is imposed by human use, such as the purpose of a fork to hold food, is called extrinsic. Natural teleology, common in classical philosophy, though controversial today, contends that natural entities also have intrinsic purposes, regardless of human use or opinion. For instance, Aristotle claimed that an acorn's intrinsic telos is to become a fully grown oak tree. Though ancient atomists rejected the notion of natural teleology, teleological accounts of non-personal or non-human nature were explored and often endorsed in ancient and medieval philosophies, but fell into disfavor during the modern era (1600–1900).

In the late 18th century, Immanuel Kant used the concept of telos as a regulative principle in his Critique of Judgment (1790).

Contemporary philosophers and scientists still debate whether teleological axioms are useful or accurate in proposing modern philosophies and scientific theories. An example of the reintroduction of teleology into modern language is the notion of an attractor. Another instance is when Thomas Nagel (2012), though not a biologist, proposed a non-Darwinian account of evolution that incorporates impersonal and natural teleological laws to explain the existence of life, consciousness, rationality, and objective value. Regardless, the accuracy can also be considered independently from the usefulness: it is a common experience in pedagogy that a minimum of apparent teleology can be useful in thinking about and explaining Darwinian evolution even if there is no true teleology driving evolution. Thus it is easier to say that evolution "gave" wolves sharp canine teeth because those teeth "serve the purpose of" predation regardless of whether there is an underlying non-teleologic reality in which evolution is not an actor with intentions. In other words, because human cognition and learning often rely on the narrative structure of stories – with actors, goals, and immediate (proximate) rather than ultimate (distal) causation (see also proximate and ultimate causation) – some minimal level of teleology might be recognized as useful or at least tolerable for practical purposes even by people who reject its cosmologic accuracy. Its accuracy is upheld by Barrow and Tipler (1986), whose citations of such teleologists as Max Planck and Norbert Wiener are significant for scientific endeavor.