Definition of sill
1
: a horizontal piece (such as a timber) that forms the lowest member or one of the lowest members of a framework or supporting structure: such as
a
: the horizontal member at the base of a window
b
: the threshold of a door
2
: a tabular body of igneous rock injected while molten between sedimentary or volcanic beds or along foliation planes of metamorphic rocks
3
: a submerged ridge at relatively shallow depth separating the basins of two bodies of water
