noun
theory
THEER-ree
noun
1
A set of ideas that explains facts or phenomena and has been supported by evidence.
"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.
"My theory is that he simply forgot about the meeting."
3
The underlying principles of a subject, as distinct from its practical application.
"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.
Common pairings
scientific theory
in theory
a theory about
Word Forms
theories plural
Study it as flashcards or scroll it in Flow — saved to your collection.
Test yourself on “theory”
A quick quiz — meaning, synonyms & usage
→
Fill the Gap
Can you complete this real example?
Darwin's _____ of evolution is supported by extensive evidence.
Etymology
From Greek theoria ("contemplation, speculation"), from theoros ("spectator").