Final answer:
The term for a finer-grained mass of material with embedded larger grains or crystals is called the matrix. It is the background material within which crystals or clasts are set.
Step-by-step explanation:
The term for a finer-grained mass of material in which larger grains, crystals, or clasts are embedded is called the matrix. In geological terms, this matrix or ground mass forms the finer-grained background in which the larger crystals, known as phenocrysts, are embedded. Porphyritic textures in igneous rocks, for instance, occur when coarse crystals are situated in a fine background which showcases a contrast in crystal sizes. Similarly, in the context of clastic rocks or composite materials, the matrix is the substance in which larger particles are dispersed.