# theory

/ˈθiɝi/ (US) · /θˈiəɹi/ (UK)

Source: https://englishreference.com/word/theory
Updated: 2026-06-19

## Senses

### 1. scientific explanation

noun · CEFR B2 · Academic

a well-tested explanation that uses facts to show how a part of the natural world works.

For teachers: a coherent set of propositions used as principles of explanation for a class of phenomena, typically confirmed through observation or experiment.

- In school, we studied the theory of evolution and how species change over time.
- The general theory of relativity fundamentally altered our understanding of space, time, and gravity within the physical sciences.

Usage: Often used with 'of' followed by the subject of study.

### 2. principles vs practice

noun · CEFR B2 · Academic

the rules and ideas of a subject, rather than the actual doing of it.

For teachers: the body of rules, ideas, and principles of a particular subject or skill as distinguished from its practical application.

- The course covers both music theory and how to play the piano.
- While the candidate understood the theory of management, he struggled with the practical realities of leading a large team.

### 3. an idea or guess

noun · CEFR B1 · Informal

an idea that someone has to explain something, even if it is not proven yet.

For teachers: a hypothesis, conjecture, or individual opinion offered as an explanation for an event.

- I have a theory that the cat only meows when she wants treats.
- Detectives are working on a theory that the two robberies were committed by the same group of people.

Teaching tip: In casual conversation, 'theory' is often used where a scientist would prefer 'hypothesis' or 'guess'.

### 4. mathematical logic

noun · CEFR C1 · Technical · Academic

a set of rules and statements that are logically connected to each other.

For teachers: a set of axioms and all the statements that can be logically derived from them within a formal system.

- The professor explained how set theory forms the basis of modern mathematics.
- In model theory, we investigate the relationship between formal languages and their interpretations in various mathematical structures.

## Examples

- The scientist developed a new theory about climate change.
- In theory, the new software should save us hours of work, but we need to test it first.
- While the general public often uses the term to describe a hunch, a scientific theory represents a framework supported by a vast body of evidence and rigorous testing.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.