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
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