A sill is a tabular sheet of igneous rock that intrudes parallel to the layering of pre-existing rocks. Sills are significant in understanding the processes of magma emplacement, the thermal effects of intrusions on surrounding rocks, and the structural geology of intrusions.
Reference: Pollard, D. D., & Johnson, A. M. (1973). “Mechanics of Growth of Some Laccolithic Intrusions in the Henry Mountains, Utah, I: Field Observations, Gilbert’s Model, Physical Properties and Flow of the Magma.” Tectonophysics, 18(3-4), 311-354.