Quick Definition: A testable series of statements that explains a phenomena
From Rational Wiki:
A scientific theory is a series of statements about the causal elements for observed phenomena. A critical component of a scientific theory is that it provides explanations and predictions that can be tested.
Usually, theories (in the scientific sense) are large bodies of work that are a composite of the products of many contributors over time and are substantiated by vast bodies of converging evidence. They unify and synchronize the scientific community’s view and approach to a particular scientific field. For example, biology has the theory of evolution and cell theory, geology has plate tectonic theory and cosmology has the Big Bang. The development of theories is a key element of the scientific method as they are used to make predictions about the world; if these predictions fail, the theory is revised. Theories are the main goal in science and no explanation can achieve a higher “rank” (contrary to the belief that “theories” become “laws” over time).
“Theory” is a Jekyll-and-Hyde term that means different things depending on the context and who is using it. While in everyday speech anything that attempts to provide an explanation for a cause can be dubbed a “theory”, a scientific theory has a much more specific meaning. Scientific theory is far more than just a casual conjecture or some Joe’s guesswork. A theory in this context is a well-substantiated explanation for a series of facts and observations that is testable and can be used topredict future observations.