Sill

Definition of Sill:

Several meanings:

A submerged bar blocking partially the mouth of a river, a harbour or an estuary.
Submerged shore-parallel dike built for the purpose of reducing wave activity at its leeside; the more usual term in this case is submerged breakwater or artificial reef.
Submerged shore-parallel dike built as support of a perched beach at the seaside.
Tabular sheet that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock ^[1].

This is the common definition for Sill, other definitions can be discussed in the article

References