theory
THEER-ree
Theory (noun) means a set of ideas that explains facts or phenomena and has been supported by evidence. Example: “The theory of gravity explains why objects fall towards the ground.”
noun
1
A set of ideas that explains facts or phenomena and has been supported by evidence.
"The theory of gravity explains why objects fall towards the ground."
"Darwin's theory of evolution is supported by extensive evidence."
"Scientists developed a new theory to explain the data."
2
A personal guess or unproven idea about why something happened.
"I have a theory that the cat is hiding because of the thunder."
"Her theory about the missing keys turned out to be wrong."
"My theory is that he simply forgot about the meeting."
3
The underlying principles of a subject, as distinct from its practical application.
"In theory the plan should work, but we have never tried it in practice."
"The course covers the theory of law before students visit a real court."
"She understands music theory but struggles to play an instrument."
How to use theory
Learner’s notesIn plain EnglishAn explanation or idea about how or why something works — can be a well-tested scientific theory or just a casual guess.
Common mistake
In science, "theory" means a well-supported explanation, not a wild guess — that everyday sense often confuses people about scientific theories like evolution.
Easily confused with
Common pairings
scientific theory
in theory
a theory about
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Word forms
- Plural
- theories
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The _____ of gravity explains why objects fall towards the ground.
Etymology
From Greek theoria ("contemplation, speculation"), from theoros ("spectator").
Synonyms
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Rhymes for theory
See all rhymes for theory →Frequently asked questions
What does “theory” mean?
The word “theory” means a set of ideas that explains facts or phenomena and has been supported by evidence. In plain terms, an explanation or idea about how or why something works — can be a well-tested scientific theory or just a casual guess.
How do you pronounce “theory”?
“Theory” is pronounced THEER-ree (/ˈθɪ.ɹi/ in IPA).
How do you use “theory” in a sentence?
Here is “theory” used in a sentence: “Darwin's theory of evolution is supported by extensive evidence.”
What part of speech is “theory”?
“Theory” is a noun.
Where does the word “theory” come from?
The word “theory” comes from Greek. From Greek theoria ("contemplation, speculation"), from theoros ("spectator").
Is “theory” a common word?
“Theory” is a very common word in modern English, usually learned at around the B1 level.
How many meanings does “theory” have?
“Theory” has 3 distinct senses listed in this dictionary. The most common is: a set of ideas that explains facts or phenomena and has been supported by evidence.
What is the difference between a theory and a hypothesis?
A hypothesis is a single, testable prediction — a proposed explanation you set out to check with an experiment. A theory is much broader: a well-established explanation supported by a large body of evidence gathered over time. In short, a hypothesis is where investigation starts, while a theory is a tested, trusted conclusion.
What is the difference between a theory and a fact?
A fact is a specific observation that can be confirmed, such as objects falling when dropped. A theory is the broader explanation of why those facts occur — the theory of gravity explains the falling. Facts are the data; a theory is the tested framework that makes sense of them.
Is a theory just a guess?
In everyday speech people say 'it's just a theory' to mean a hunch, but in science the word means the opposite. A scientific theory is one of the strongest forms of knowledge — an explanation tested repeatedly and supported by extensive evidence. It is far more reliable than a casual guess.
What is an example of a scientific theory?
Well-known examples include the theory of evolution, which explains how species change over generations, and the theory of relativity, which describes space, time, and gravity. Each began as an explanation of observed facts and became a theory only after decades of testing and confirming evidence.
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