Explanatory variable

The explanatory variable, also called the independent or predictor variable, is the one you treat as the cause or predictor in a relationship. It sits on the horizontal axis and supplies the $x$ values in a regression. For example, when studying how fertilizer amount affects plant height, fertilizer is the explanatory variable and height is the response. Labeling a variable explanatory reflects the question you are asking, and in an observational study it does not by itself prove that variable causes the response.