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
A statement that explains observations and can be tested is called a hypothesis. It serves as a proposed explanation for a phenomenon and is formulated based on existing knowledge or observations. A hypothesis can be tested through experiments or further observation to determine its validity.
A statement that explains an observation and is supported by data is a
A system of ideas that explains many related observations and is supported by a large body of scientific evidence is called a theory. In science, theories integrate and interpret a wide range of data, providing a framework for understanding phenomena and making predictions. They are continually tested and refined based on new evidence, distinguishing them from mere hypotheses. Examples include the theory of evolution and the theory of relativity.
A scientific explanation for a set of observations that can be tested in ways that support it or reject it is called a hypothesis. A hypothesis is a proposed explanation for a phenomenon that can be tested through experiments or observations to determine its validity.
Scientific Method
Direct observations. This involves directly watching or recording the behavior or occurrence without any inference or interpretation.
scientific method
A statement that explains how and why specific facts are related is called a hypothesis. A hypothesis provides a testable explanation for observations and helps guide research by predicting relationships between variables. It serves as a foundational element in the scientific method, allowing researchers to explore and validate theories through experimentation and analysis.
A theory. (And sometimes called a law.)
data the recorded observations are called data.