Main Page | See live article | Alphabetical index In geology, a sill is a tabular mass of igneous rock that has been intruded laterally between layers of sedimentary rock, beds of volcanic lava or ejectmenta, or even along the direction of the foliation in metamorphic rock. The term "sill" is synonymous with "intrusive sheet." In architecture, a sill is the bottom edge of a window. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.