theory
Pluraltheories
- 1An idea or explanation put forward for why something happens or how something works, especially one that has not been proved.
She had a theory that the missing keys had slipped down behind the car seat.
His theory about the strange noise turned out to be completely wrong.
UsageOften have a theory that + clause, or a theory about something.
Similarhypothesis, idea, notion, explanation, guess
- 2A coherent set of ideas that explains observed facts, correctly predicts new ones, and has been confirmed by observation and experiment.
Darwin's theory of evolution explains how species change over time.
Once enough evidence supported it, the theory became widely accepted among researchers.
Similarexplanation, model
Note
The scientist's sense, and the strongest one: a theory here is the product of extensive testing, not a guess, though it stays open to revision. John Stuart Mill called it "the completed result of philosophical induction from experience," and Bill Bryson describes the Big Bang moving "from interesting idea to established theory."
- 3The principles and established methods of a subject, skill, art, or game, as opposed to putting them into practice.
He understood the theory of painting long before he picked up a brush.
In medical training, theory and hands-on practice are taught side by side.
UsageOften paired with practice: theory and practice. Say music theory or the theory of music.
Oppositepractice
Note
The sense that comes paired with practice: what you can know about an art as distinct from doing it.
- 4A field of study that attempts to describe a particular class of constructs as fully as possible.
Mathematicians developed an entire theory to describe this class of objects.
Number theory concerns itself with the deep properties of the integers.
UsageField names often put the subject before theory, without an article: number theory, knot theory. Say work in number theory.
Note
Mathematics names whole disciplines this way. Knot theory, for instance, classifies the ways a circle can be embedded in three-dimensional space.
Less common meanings
- 5A set of axioms together with all the statements that can be derived from them.
Every theory in first-order logic is closed under logical consequence.
They proved that the theory was consistent.
Note
In logic the word drops every suggestion of evidence or confirmation that clings to the scientific sense. A theory is simply what a set of axioms commits you to: the axioms plus everything that follows from them. Its virtues are formal ones. Logicians ask whether a theory is consistent, and a theory is consistent if it has a model. Because the notion is exact, two theories can be compared precisely, for instance by asking whether one proves everything the other does.
- 6The mental act of conceiving something; reflection or careful consideration.
Note
The meaning that stays closest to the Greek. Theōría meant "contemplation, a looking at," from theōrós, a spectator, and théa, a view. Here theory is the act of thinking something over, not any account that thinking produces. Because the sense is obsolete, a reader will meet it only in older writing, where a theory of something can mean careful reflection on it, with no claim to explain anything.
Checked 1 Nov 2026 · 4 facts
- Greek theōría meant contemplation or looking at. en.wikipedia.org
- Theōría derives from theōrós, meaning spectator. en.wiktionary.org
- Théa means view and is an ultimate root of theōrós. en.wiktionary.org
- Theory has an obsolete sense meaning mental conception, reflection, or consideration. en.wiktionary.org
Practice
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Origin
The word came through Middle French théorie and Late Latin theōria from Ancient Greek θεωρία (theōría, "contemplation, speculation, a looking at"), which derives from θεωρέω (theōréō, "I look at, view, consider") and ultimately from θεωρός (theōrós, "spectator"), formed from θέα (théa, "view, sight") and ὁράω (horáō, "I see, look").
Checked 1 Nov 2026 · 5 facts
- English theory was borrowed through Middle French théorie and Late Latin theōria from Ancient Greek θεωρία. en.wiktionary.org
- Theōría meant contemplation, speculation, or looking at. en.wikipedia.org
- Theōréō means I look at, view, or consider. en.wiktionary.org
- Theōréō derives from theōrós, meaning spectator. en.wiktionary.org
- Theōrós is formed from the roots of théa, view or sight, and horáō, I see or look. en.wiktionary.org