Charles Darwin is credited with developing a persuasive argument for the theory of evolution.
The word “theory” means several different things, depending on the context. In math and science, for example, it is a tried and tested concept that is used to explain an occurrence. For students of the arts, the term refers to the non-practical aspect of their work, while laypeople refer to unproven ideas and speculations as theories. The plethora of meanings for this word can get confusing, but the intent is usually clear from the context; a mathematical article talking about a theory, for example, is probably referring to it in the scientific sense.
Isaac Newton’s theory of gravity was tested and proved to be correct.
In English, the word dates back to 1592, when it was used to mean a concept or scheme. In the 1630s, scientists co-opted the term, using it to describe an explanation or thought based on observation and testing. “Theorizing” also appeared at about the same time.
In the sciences, theories are created after observation and testing. They are designed to explain a phenomenon rationally and clearly. For example, Isaac Newton came up with a theory about gravity in the 17th century, and it proved to be testable and correct. Scientific theories are not quite the same thing as facts, but they are often very similar; Scientists often test their theories extensively before expounding them, looking for obvious problems that might cause them to be questioned.
In mathematics, theories are bodies of knowledge about specific types of mathematics. Mathematicians use things like set theory in the course of their work. Theoretical math can be quite complex and abstract, making it sometimes difficult for laypeople to understand, but it helps explain everything from the movement of crowds to the origins of the universe.
In the arts, many artists refer to their non-practical work as theoretical. For example, a musician who plays the tuba would use the term to refer to the study of music history, music mathematics, and other related materials. Art criticism is also a field of theory, as critics discuss the work of art, rather than actively producing it, and through their discussions contribute to the general field.
For laymen, a theory is simply an idea. Some people use the word as if it were “hypothesis”, postulating an idea that needs to be tested. Other times, an idea may be dismissed as “just a theory”, with the implication that it cannot be proved and is only a rough idea, not a fact or firm opinion.