Definition of sill

The term "sill" originally derived from an Old Norse word, "slēll," which referred to the bottom step or threshold of a door or entryway. Over time, in the context of masonry and construction, the meaning of the word evolved to describe a horizontal, supporting member beneath a window, door, or other opening. This use of the term "sill" can be traced back to the Middle English language, where it was spelled "sel" or "syll." By the late 1400s, the spelling had settled into its current form, with a double "l" to reflect its pronunciation. Today, the word "sill" appears in various contexts, including in discussions of architecture, geology, and biology, each with their own unique definitions depending on the specific use case.