It is called a Scientific Theory.
An idea that explains something could be called a theory. Good scientific theories are falsifiable which means you could prove it to be false. You can never prove a theory to be true, you can only provide evidence that supports the theory.
The steps in an experiment are called the Scientific Method.
Results
There was no middle class in Europe in the middle ages. People were either very rich or very poor. Towards the late middle ages a merchant class did develop, but this couldn't be called "middle class". The concept of a middle class is a modern idea and actually began to develop after the industrial revolution.
Scientific theory
If by "why" you mean "what," a well tested framework that explains a wide range of observations in science is called a theory.
hypothesis
Indirect.
Scientific Method
Scientific Method
scientific method
A theory. (And sometimes called a law.)
data the recorded observations are called data.
Make observations.
A theory. (And sometimes called a law.)
A brief statement summarizing many observations of a physical phenomenon is called scientific law. The density of an object is the ratio of its mass to volume.
the scientific method is called a deductive approachconclusionPrediction