The principle that identifies the nature of the relationship between a cause and its effect. I frequently use this formulation. The nature of the existent that acts, moves, changes or endures, causes the nature of that existent's action, motion, change or endurance to be what it is.1 For related elaboration, please see PRINCIPLE (FUNDAMENTAL, PHILOSOPHICAL).
Elaboration

As do my fundamental, philosophical principles of reality, identity and non-contradiction, in my terms, the principle of causality provides a different vantage point on the fact that what is, is.

To clarify how, for me, the principle of causality restates those of reality, identity and non-contradiction, it's satisfied me to restate the principle of causality this way: The nature of an existent causes the nature of that existent's being to be what it is. In my terms, what is, is (reality); what a thing is, is what it is (identity); in a specific respect and at a specific instant, a thing is what it is, and is not what it's not (non-contradiction); and what a thing is causes it to be what it is (causality). (For me, action, motion, change and endurance are particular types of being, so "what a thing is causes it to be what it is" captures the same, basic meaning as does "the nature of the existent that acts, moves, changes or endures, causes the nature of that existent's action, motion, change or endurance to be what it is.") By my standards, each such principle implies each of the others and each provides a different focus on the same reality.

Also in my terms, now emphasizing the different focus provided by each principle, just as the principle of identity focuses on a thing's limits, so the principle of causality focuses on its freedoms.

By my epistemological standards, the concept "causality" ultimately can be defined only ostensively. See DEFINITION (OSTENSIVE)

Notes

1I drew inspiration from Ayn Rand's discussion of causality in her novel, Atlas Shrugged when I first formulated my own principle of causality. With additional reflection, I have evolved my formulation beyond such origins.