Explanans and explanandum

A part of the formal structure of scientific explanation.  The explanandum `or conclusion is the fact to be explained, while the explanans is the explanatory account or argument that rationalizes the explanandum (i.e., an account used to answer the why question).  This distinction provides the means for identifying different types of scientific explanations. 

See Deductive-nomological model, Explanation, Prediction and explanation