theory

theory — noun

1. a set of well-tested ideas that explains why something happens in nature or soci

1.nounB1
Definition

a set of well-tested ideas that explains why something happens in nature or society, based on evidence and careful study

Example

Darwin's theory of evolution explains how species gradually adapt to changing environments.

collocation: theory of [something]

Noa tested the theory by measuring water temperature at different depths.

transitive use: test a theory

Common collocations
  • develop a theory
  • propose a theory
  • test a theory
  • support a theory
  • theory of evolution
  • theory of relativity
  • according to theory
  • current theory
Synonyms
  • hypothesis

    a proposed but not yet fully tested explanation — narrower and more tentative than a theory

  • model

    a simplified representation used to analyse or predict a system — often more concrete and specific

  • framework

    a basic structure of ideas that supports further thinking — broader and less formal than a scientific theory

  • doctrine

    a set of beliefs held and taught by a group — often implies authority or ideology rather than empirical testing

Grammar Patterns

theory of [something]

theory that [clause]

[verb] + theory

Usage Note

Commonly constructed with 'of' ('theory of relativity') or a 'that'-clause ('the theory that...'). In everyday English, 'theory' is sometimes used loosely to mean 'an untested guess,' but in academic and scientific contexts it refers to a well-supported explanatory framework supported by evidence.

Common Mistakes

Evolution is just a theory, not a proven fact.
Evolution is a scientific theory supported by overwhelming evidence.
In science, a theory is a well-tested explanation, not a casual guess. Using 'just' before 'theory' undermines its actual meaning in scientific contexts.

2. applies when something should be possible according to an idea or system, but in

2.nounB2
Definition

applies when something should be possible according to an idea or system, but in reality it often works out differently

Example

In theory, the faster computer should finish the task in half the time.

adverbial phrase: 'in theory' at start of sentence

The new policy makes sense in theory, but it is hard to put into practice.

contrast pattern: 'in theory, but...'

Common collocations
  • in theory
  • in theory but in practice
  • sound good in theory
  • work in theory
  • possible in theory
Synonyms
  • in principle

    similar meaning but slightly more formal, focusing on the underlying concept rather than a hypothetical scenario

  • theoretically

    adverb form that can replace the phrase 'in theory' in most contexts

Antonyms
  • in practice

    the most direct contrast — what actually happens versus what is supposed to happen

  • in reality

    similar to 'in practice', emphasizing the real-world outcome rather than the expected one

Grammar Patterns

in theory

in theory ... but in practice / in reality

Usage Note

The phrase 'in theory' is the most common form of this sense; the adverb 'theoretically' can be used in the same way ('Theoretically, the machine should work'). The contrast pattern 'in theory...but in practice/reality...' is especially frequent in discussions of plans, policies, and proposals.

Common Mistakes

The plan works in theory but fails on practice.
The plan works in theory but fails in practice.
The fixed expression is 'in practice,' not 'on practice,' when contrasting with 'in theory.'
Idioms

in theory

idiom

Used to describe something that is believed to be true based on ideas, but may not happen in real life.

In theory, the new software should reduce errors by half.

That plan sounds good in theory, but will it work here?

Word family

Synonym discussion

Sense: noun/1

A theory is the most established of these terms in science — it explains a broad range of phenomena with strong evidence. A hypothesis is a narrower, untested proposition that needs verification (e.g. 'the hypothesis that caffeine improves memory'). A model is a simplified representation, often mathematical or visual, used to simulate a system (e.g. 'a climate model'). A framework is a looser set of concepts that structures how a topic is approached (e.g. 'a framework for ethical decision-making'). A doctrine belongs more to ideology or religion than empirical science. In daily language, 'hypothesis' is often used where 'theory' would be the more accurate scientific word, while 'framework' has become common in social sciences and business writing.

hypothesis ↗model ↗framework ↗doctrine ↗

Common collocations

Nouns

Adverbs

Phrases

  • in theory
  • according to theory
  • based on a theory

Prepositions

Etymology

GreekLatinFrenchEnglish

The word 'theory' traces back to the ancient Greek word 'theoria' (θεωρία), which meant 'contemplation' or 'a looking at.' It came from 'theoros' (spectator), which combined 'thea' (a view) and 'horan' (to see). The idea was that a theory involved stepping back to observe and reflect, rather than just taking action. The word passed into Latin as 'theoria,' then into French as 'théorie,' and finally entered English in the late 1500s with the meaning of 'a conception or mental scheme.' Over time it developed its modern scientific sense of a tested, explanatory system.