Theory
Part of speech: noun
Definition: An unproven conjecture. An expectation of what should happen, barring unforeseen circumstances. A coherent statement or set of statements that attempts to explain observed phenomena. A logical structure that enables one to deduce the possible results of every experiment that falls within its purview. A field of study attempting to exhaustively describe a particular class of constructs. A set of axioms together with all statements derivable from them.
Opinion
Part of speech: noun
Definition: A thought that a person has formed about a topic or issue.
Part of speech: verb
Definition: To have or express as an opinion.