theory Definition

  • 1a formal set of ideas that is intended to explain why something happens or exists
  • 2an idea or set of ideas that is intended to explain something about life or the world, especially one that has not yet been proven true

Using theory: Examples

Take a moment to familiarize yourself with how "theory" can be used in various situations through the following examples!

  • Example

    The theory of relativity revolutionized physics.

  • Example

    There are many theories about the origin of the universe.

  • Example

    The theory that laughter is contagious has been supported by scientific studies.

  • Example

    The conspiracy theory that the moon landing was faked has been debunked.

theory Synonyms and Antonyms

Synonyms for theory

Antonyms for theory

Idioms Using theory

  • in both the abstract and practical aspects of something


    The new teaching method sounded great in theory, but in theory and practice, it didn't work as well as expected.

  • to suggest an idea or explanation for something


    She put forward a theory that the company's financial troubles were due to mismanagement.

  • to have an idea or explanation for something, often without much evidence to support it


    I have a theory that the missing keys are somewhere in the living room.

Phrases with theory

  • according to what is generally expected or believed to be true, although it may not be true in reality


    In theory, the new system should be more efficient, but we'll have to wait and see.

  • conspiracy theory

    a belief that an event or situation is the result of a secret, often sinister, plot by a group of people


    The conspiracy theory that the government is hiding evidence of extraterrestrial life is popular among some groups.

  • the study of mathematical models of strategic interaction among rational decision-makers


    Game theory is used in economics, political science, and psychology to analyze decision-making.

Origins of theory

from Greek 'theoria', meaning 'contemplation, speculation'


Summary: theory in Brief

The term 'theory' [ˈθɪəri] refers to a formal set of ideas or an idea that explains why something happens or exists. It can range from scientific theories like the theory of relativity to unproven beliefs like conspiracy theories. Phrases like 'in theory' and idioms like 'put forward a theory' are common. 'Theory' is often used in contrast to 'fact' or 'reality.'

How do native speakers use this expression?