The Humean mosaic refers to totality of all facts and properties of the world in four-dimensional spacetime, within which there is no metaphysical distinction between scientific laws and other propositions.
This view is invoked in the descriptivist account of scientific laws, which takes laws to supervene on particular facts about the world. David Lewis refers to this relationship between laws and facts as Humean supervenience:
“All there is to the world is a vast mosaic of local matters of particular fact, just one little thing and another” (1986b: ix).
Descriptivism has the specific advantage of giving a clear epistemological account of how science can discover laws in the first place: we can read law-like statements directly from particular observations.
Related notes: Best system analysis of natural laws, after Lewis