Definitions for theoryˈθi ə ri, ˈθɪər i

Random House Webster's College Dictionary

the•o•ry^*ˈθi ə ri, ˈθɪər i(n.)(pl.)-ries.

a coherent group of general propositions used as principles of explanation for a class of phenomena:
Darwin's theory of evolution.
a proposed explanation whose status is still conjectural.
a body of mathematical principles, theorems, or the like, belonging to one subject:
number theory.
Category: Math
the branch of a science or art that deals with its principles or methods, as distinguished from its practice:
music theory.
a particular conception or view of something to be done or of the method of doing it.
a guess or conjecture.
contemplation or speculation.

Idioms for theory:

in theory,under hypothetical or ideal conditions; theoretically.
Category: Idiom

^* Syn: 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: Einstein's 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.

Origin of theory:

1590–1600; < LL theōria < Gk theōría observing, contemplation, theory =theōr(eîn) to observe (see theorem ) +-ia-y3

Princeton's WordNet

theory(noun)
a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena
"theories can incorporate facts and laws and tested hypotheses"; "true in fact and theory"
hypothesis, possibility, theory(noun)
a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena
"a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices"
theory(noun)
a belief that can guide behavior
"the architect has a theory that more is less"; "they killed him on the theory that dead men tell no tales"

Kernerman English Learner's Dictionary

theory(noun)ˈθi ə ri, ˈθɪər i
an explanation sb believes is true, although there is no proof
My theory is that she's secretly in love with you.; a theory about who was behind the attack
theoryˈθi ə ri, ˈθɪər i
in science, an idea or set of ideas that attempt to explain how or why sth happens
the theory of Evolution
theoryˈθi ə ri, ˈθɪər i
all the ideas or rules on which a subject is based
literary theory
theoryˈθi ə ri, ˈθɪər i
an idea of the best way to do sth, which may be different from what happens in reality
In theory, when rich people get richer, the rest of society should benefit.

Wiktionary

theory(Noun)
Mental conception; reflection, consideration.
theory(Noun)
A coherent statement or set of ideas that explains observed facts or phenomena, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc.
theory(Noun)
The underlying principles or methods of a given technical skill, art etc., as opposed to its practice.
theory(Noun)
A field of study attempting to exhaustively describe a particular class of constructs.
Knot theory classifies the mappings of a circle into 3-space.
theory(Noun)
A hypothesis or conjecture.
theory(Noun)
A set of axioms together with all statements derivable from them. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems).
A theory is consistent if it has a model.

Webster Dictionary

Theory(noun)
a doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation
Theory(noun)
an exposition of the general or abstract principles of any science; as, the theory of music
Theory(noun)
the science, as distinguished from the art; as, the theory and practice of medicine
Theory(noun)
the philosophical explanation of phenomena, either physical or moral; as, Lavoisier's theory of combustion; Adam Smith's theory of moral sentiments

The New Hacker's Dictionary

theory
The consensus, idea, plan, story, or set of rules that is currently being used to inform a behavior. This usage is a generalization and (deliberate) abuse of the technical meaning. “What's the theory on fixing this TECO loss?” “What's the theory on dinner tonight?” (“Chinatown, I guess.”) “What's the current theory on letting lusers on during the day?” “The theory behind this change is to fix the following well-known screw....”