theory

UK: /ˈθiːəri/ US: /ˈθiəri, ˈθɪri/ ,(thē′ə rē, thēr′ē)

Inflections of 'theory' (n): npl: theories

WordReference Random House Learner's Dictionary of American English © 2025

the•o•ry /ˈθiəri, ˈθɪri/   n., pl. -ries. 

1. Science. a group or collection of general statements that together as principles explain some fact or group of facts:
   [countable] Darwin's theory of evolution; quantum theory in physics.
2. Mathematics a body or collection of mathematical principles belonging to one subject:
   [uncountable] the study of number theory.
3. the branch of a science or art that deals with its principles or methods, as distinguished from its practice:
   [uncountable] He took courses in music theory.
4. an explanation for some fact or behavior claimed to be true but not yet proven:
   [countable] He has a theory about why so many students have trouble with the word the in English.

Idioms

5. Idioms in theory, under ideal conditions;
   theoretically:
   In theory everyone has certain equal rights.

WordReference Random House Unabridged Dictionary of American English © 2025

the•o•ry  (thē′ə rē, thēr′ē), n., pl. -ries. 

1. a coherent group of general propositions used as principles of explanation for a class of phenomena:
   Einstein's theory of relativity.
2. a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.
3. Mathematics a body of principles, theorems, or the like, belonging to one subject:
   number theory.
4. the branch of a science or art that deals with its principles or methods, as distinguished from its practice:
   music theory.
5. a particular conception or view of something to be done or of the method of doing it;
   a system of rules or principles.
6. contemplation or speculation.
7. guess or conjecture.

• Greek theōría a viewing, contemplating, equivalent. to theōr(eîn) to view + -ia -y³
• Late Latin theōria
• 1590–1600

Synonyms

• 1. Theory, hypothesis are used in non-technical contexts to mean an untested idea or opinion. A theory in technical use is a more or less verified or established explanation accounting for known facts or phenomena:
  the theory of relativity.
  A hypothesis is a conjecture put forth as a possible explanation of phenomena or relations, which serves as a basis of argument or experimentation to reach the truth:
  This idea is only a hypothesis.

Collins Concise English Dictionary © HarperCollins Publishers::

theory / ˈθɪərɪ/ noun ( -ries)

1. a system of rules, procedures, and assumptions used to produce a result
2. abstract knowledge or reasoning
3. a speculative or conjectural view or idea: I have a theory about that
4. an ideal or hypothetical situation (esp in the phrase in theory)
5. a set of hypotheses related by logical or mathematical arguments to explain and predict a wide variety of connected phenomena in general terms: the theory of relativity
6. a nontechnical name for hypothesis1

Etymology: 16th Century: from Late Latin theōria, from Greek: a sight, from theōrein to gaze upon