Simply put: philosophy is the act of defining (or attempting to define) a belief system or mental model - usually, with the goal of contextualizing or understanding our lived experience, or in order to facilitate communication about a particular problem space or field of inquiry.

Note that 'philosophy' only refers to the act of creating the belief system. It does not refer to any subsequent acts of discovery made by following the rules established by the system.

As an example, the formulation of Euclidean geometry would be an act of philosophy. But the act of proving, for example, the Pythagorean theorem, within the pre-established framework of Euclidian geometry would not be philosophy.