Reality is the actuality of something. The term is used to denote what is real, as opposed to what is merely imaginary. The study of reality is called ontology. Philosophers debate whether “reality” exists independently of human perception, and also whether it can be known with certainty. In practical terms, “reality” often refers to the empirical world: things that can be observed and measured. It may also refer to an absolute reality, such as the way things actually are, independent of observation or measurement. Scientific theories are usually intended to explain some aspect of reality; they are not themselves part of reality (although sometimes they may become so).
There are different ways of understanding what “reality” means. One common approach is scientific realism: the view that the world described by science is really out there, independent of our thoughts and perceptions. This perspective was advocated by many scientists in the past, including Isaac Newton and Albert Einstein. But it has come under attack in recent years from philosophers and historians who argue that scientific theories are inevitably shaped by the culture and values of their time – in other words, they are not really about objective reality at all.
A related idea is philosophical realism, which holds that there is an objective reality that we can know about (albeit imperfectly). This view has been defended by thinkers such as Plato and René Descartes. Philosophical realism generally takes a more skeptical stance towards claims about what we can know about reality; it emphasizes fallibility and caution in making claims about what exists outside our own minds.
The question of what “reality” really means remains a matter of ongoing debate among philosophers, scientists and other thinkers.