theory

The word “theory” means a set of ideas that explains facts or phenomena and has been supported by evidence. In plain terms, an explanation or idea about how or why something works — can be a well-tested scientific theory or just a casual guess.

“Theory” is pronounced THEER-ree (/ˈθɪ.ɹi/ in IPA).

Here is “theory” used in a sentence: “Darwin's theory of evolution is supported by extensive evidence.”

“Theory” is a noun.

The word “theory” comes from Greek. From Greek theoria ("contemplation, speculation"), from theoros ("spectator").

“Theory” is a very common word in modern English, usually learned at around the B1 level.

“Theory” has 3 distinct senses listed in this dictionary. The most common is: a set of ideas that explains facts or phenomena and has been supported by evidence.

A hypothesis is a single, testable prediction — a proposed explanation you set out to check with an experiment. A theory is much broader: a well-established explanation supported by a large body of evidence gathered over time. In short, a hypothesis is where investigation starts, while a theory is a tested, trusted conclusion.

A fact is a specific observation that can be confirmed, such as objects falling when dropped. A theory is the broader explanation of why those facts occur — the theory of gravity explains the falling. Facts are the data; a theory is the tested framework that makes sense of them.

In everyday speech people say 'it's just a theory' to mean a hunch, but in science the word means the opposite. A scientific theory is one of the strongest forms of knowledge — an explanation tested repeatedly and supported by extensive evidence. It is far more reliable than a casual guess.

Well-known examples include the theory of evolution, which explains how species change over generations, and the theory of relativity, which describes space, time, and gravity. Each began as an explanation of observed facts and became a theory only after decades of testing and confirming evidence.