It is a plausible and consistent explanation for observable phenomena.
To be considered scientific, a theory must be falsifiable. This means that there must be some way to do experiments that could 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.  Scientific theories can sometimes be used as the basis for industrial and technical developments.
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.
Although Science can not in actuality prove anything to be true for sure all it can say is the the preponderance of evidence shows that to be the most likely answer.
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 science a theory is something that ties together facts. For example, an apple falling to the ground when you let go of it is a fact. From this we can draw the conclusion that there is some force pulling it down. This theory is called the theory of gravity.
Notes and references
- For example, scientists may refer to phlogiston theory or ether theory, even though empirical verification was lacking.
- "A theory not only explains known facts; it also allows scientists to make predictions of what they should observe if a theory is true. Scientific theories are testable. New evidence should be compatible with a theory. If it isn't, the theory is refined or rejected."