Theory Definition in Science

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The definition of a theory in science is very different from the everyday usage of the word. In fact, it's usually called a "scientific theory" to clarify the distinction. In the context of science, a theory is a well-established explanation for scientific data. Theories typically cannot be proven, but they can become established if they are tested by several different scientific investigators. A theory can be disproven by a single contrary result.

Key Takeaways: Scientific Theory

  • In science, a theory is an explanation of the natural world that has been repeatedly tested and verified using the scientific method.
  • In common usage, the word "theory" means something very different. It could refer to a speculative guess.
  • Scientific theories are testable and falsifiable. That is, it's possible a theory might be disproven.
  • Examples of theories include the theory of relativity and the theory of evolution.


There are many different examples of scientific theories in different disciplines. Examples include:

Key Criteria for a Theory

There are certain criteria which must be fulfilled for a description to be a theory. A theory is not simply any description that can be used to make predictions!

A theory must do all of the following:

  • It must be well-supported by many independent pieces of evidence.
  • It must be falsifiable. In other words, it must be possible to test a theory at some point.
  • It must be consistent with existing experimental results and able to predict outcomes at least as accurately as any existing theories.

Some theories may be adapted or changed over time to better explain and predict behavior. A good theory can be used to predict natural events that have not occurred yet or have yet to be observed.

Value of Disproven Theories

Over time, some theories have been shown to be incorrect. However, not all discarded theories are useless.

For example, we now know Newtonian mechanics is incorrect under conditions approaching the speed of light and in certain frames of reference. The theory of relativity was proposed to better explain mechanics. Yet, at ordinary speeds, Newtonian mechanics accurately explains and predicts real-world behavior. Its equations are much easier to work with, so Newtonian mechanics remains in use for general physics.

In chemistry, there are many different theories of acids and bases. They involve different explanations for how acids and bases work (e.g., hydrogen ion transfer, proton transfer, electron transfer). Some theories, which are known to be incorrect under certain conditions, remain useful in predicting chemical behavior and making calculations.

Theory vs. Law

Both scientific theories and scientific laws are the result of testing hypotheses via the scientific method. Both theories and laws may be used to make predictions about natural behavior. However, theories explain why something works, while laws simply describe behavior under given conditions. Theories do not change into laws; laws do not change into theories. Both laws and theories may be falsified but contrary evidence.

Theory vs. Hypothesis

A hypothesis is a proposition which requires testing. Theories are the result of many tested hypotheses.

Theory vs Fact

While theories are well-supported and may be true, they are not the same as facts. Facts are irrefutable, while a contrary result may disprove a theory.

Theory vs. Model

Models and theories share common elements, but a theory both describes and explains while a model simply describes. Both models and theory may be used to make predictions and develop hypotheses.


  • Frigg, Roman (2006). "Scientific Representation and the Semantic View of Theories." Theoria. 55 (2): 183–206. 
  • Halvorson, Hans (2012). "What Scientific Theories Could Not Be." Philosophy of Science. 79 (2): 183–206. doi:10.1086/664745
  • McComas, William F. (December 30, 2013). The Language of Science Education: An Expanded Glossary of Key Terms and Concepts in Science Teaching and Learning. Springer Science & Business Media. ISBN 978-94-6209-497-0.
  • National Academy of Sciences (US) (1999). Science and Creationism: A View from the National Academy of Sciences (2nd ed.). National Academies Press. doi:10.17226/6024 ISBN 978-0-309-06406-4. 
  • Suppe, Frederick (1998). "Understanding Scientific Theories: An Assessment of Developments, 1969–1998." Philosophy of Science. 67: S102–S115. doi:10.1086/392812
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Helmenstine, Anne Marie, Ph.D. "Theory Definition in Science." ThoughtCo, Aug. 28, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 28). Theory Definition in Science. Retrieved from Helmenstine, Anne Marie, Ph.D. "Theory Definition in Science." ThoughtCo. (accessed April 10, 2021).