"theory" - key terms from Wikipedia

attachment theory
big bang theory
chaos theory
color theory
communication theory
computability theory
computational complexity theory
conspiracy theory
control theory
critical theory
decision theory
dependency theory
economic theory
evolutionary theory
feminist theory
field theory
galois theory
game theory
gauge theory
graph theory
grounded theory
group theory
hybrid theory
information theory
international relations theory
kinetic theory
learning theory
literary theory
model theory
music theory
number theory
perturbation theory
political theory
probability theory
quantum field theory
quantum theory
rational choice theory
scientific theory
set theory
social theory
string theory
systems theory
theory
theory	of a deadman
theory	of everything
theory	of evolution
theory	of justice
theory	of mind
theory	of relativity
valence bond theory