A sill is a sheet intrusion emplaced parallel to the structures present within its host rocks. Its primary feature is to be concordant and never crosscut, indeed, the foliation or bedding planes in the host, which magma exploits as planes of weakness to move underground. Sills are typically tabular (i.e. table-like), developing mainly along 2 directions and showing limited thickness. A dike is also a tabular intrusion but, contrarily to sills, is discordant with the structures of the host. Sills may laterally evolve to dikes or vice-versa and split into several dikes and sills. Sills are usually fed by diked (feeder dikes) that are linked to other magmatic intrusions or to the magma source at depth. Large concentrations of sills in the same area that share common characteristics like age, composition, and orientation are sill swarms or sill complexes.

At outcrop scale, sills are parallel with layers or foliations above and below and may be confused with lava flows interlayered within sedimentary beds or volcanic rocks. However, they are different from lava flows as they show intrusive contacts on both sides. They may contain xenoliths originated from the overlying and underlying rocks. Contact metamorphism occurring again above and below the sill is another evidence for intrusive emplacement. For the same reason, a sill may show evidence of fast cooling (e.g. chilled margin) at the contact with all the surrounding country rocks in the same way. On the other hand, a lava flow tend to show evidence of very rapid cooling and exposure to atmospheric agents only on the upward side.
Care is needed in metamorphic sequences, since metamorphism may destroy these evidences, making it difficult to distinguish a metamorphosed sill from a metamorphosed lava flow.