A theory is an abstract or systematic explanation of some phenomenon or subject matter. A scientific theory is more substantial than a conjecture or hypothesis, but may not be verified as consistent with empirical data. 
The term theory is often used outside of science, such as in music theory, number theory, or Marxist theory.
A theory is a plausible and consistent explanation for observable phenomena. A scientific theory is a model or framework for describing a related set of natural or social behaviors or observations. A scientific theory must be falsifiable, meaning that there must be some way to do experiments that counter the theory's predictions, thus disproving the current theory. Generally accepted scientific theories have been tested and survived over time, have evolved when appropriate, and modified toward consistency with newly discovered data, have not been shown to be false and can make predictions about natural phenomena. 
A scientific theory does not necessarily have to have strong experimental support or accepted by the scientific community. Scientists often refer to untested theories and competing theories. Theories can be extremely well-confirmed, such as conservation of energy, or speculative, such as String Theory.
The Theory of Evolution includes microevolution (change in allele frequency over time), which is well-confirmed, and the idea that there existed a universal common ancestor for all life on Earth, which remains controversial.[Citation Needed]
Evolutionists frequently argue that the word "theory" means very different things to scientists and non-scientists. For example, a PBS TV show says: 
- When we use the word "theory" in everyday life, we usually mean an idea or a guess, but the word has a much different meaning in science. This video examines the vocabulary essential for understanding the nature of science and evolution and illustrates how evolution is a powerful, well-supported scientific explanation for the relatedness of all life.
The American Museum of Natural History exhibit on Darwin says: 
- In everyday use, the word "theory" often means an untested hunch, or a guess without supporting evidence. But for scientists, a theory has nearly the opposite meaning. A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts.
In fact, it is hard to find anyone who makes this distinction, outside those who are promoting the theory of evolution to the general public. 
Notes and references
- For example, scientists may refer to phlogiston theory or ether theory, even though empirical verification was lacking.
- For example, this 2004 NY Times article on the pros and cons of string theory  used the word "theory" 70 times without any suggestion that ideas must be well-substantiated before they qualify as a theory. Physicist Lawrence Krauss used the term "string theory" even though the theory has "yet to have any real successes in explaining or predicting anything measurable".  A month later, he argued that it should really be called the "string enterprise" because using the word "theory" causes problems with intelligent design advocates and those who say that evolution is "just a theory".