Did you know?

Click any word in a definition or example to find the entry for that word

theory - definition and synonyms

What are red words?

90% of the time, speakers of English use just 7,500 words in speech and writing. These words appear in red, and are graded with stars. One-star words are frequent, two-star words are more frequent, and three-star words are the most frequent.

Thesaurus diagram

The thesaurus of synonyms and related words is fully integrated into the dictionary. Click on the thesaurus category heading under the button in an entry to see the synonyms and related words for that meaning.

noun     theory pronunciation in British English
Word Forms
  1. 1
    [countable] one or more ideas that explain how or why something happens

    The theory predicts low speeds and therefore safety.

    theory that:

    He had a theory that the germs caused disease.

  2. 2
    [uncountable] the set of general principles that a particular subject is based on

    the theory and practice of education

    psychoanalytic/Marxist/literary theory

  3. 3
    [countable] an idea that you believe is true although you have no proof
    theory about:

    I have my own theory about why he resigned.

    theory that:

    There is a theory that her death was an accident.




a lifestyle focussing on simple pleasures such as comfort and cosiness in the home, and spending time with friends and family

BuzzWord Article

Open Dictionary


a form of location that involves the underwater detonation of a bomb which causes sound waves that are picked up by ships

add a word


A must for anyone with an interest in the changing face of language. The Macmillan Dictionary blog explores English as it is spoken around the world today.

global English and language change from our blog
Macmillan learn live love play