theory /ˈθiːjəri/ noun
plural theories
plural theories
Learner's definition of THEORY
: an idea or set of ideas that is intended to explain facts or events
[count] : an idea that is suggested or presented as possibly true but that is not known or proven to be true
[noncount] : the general principles or ideas that relate to a particular subject

in theory

used to say what should happen or be true if a theory is correct
used to say that something seems to be true or possible as an idea but may not actually be true or possible