sill

UK: /ˈsɪl/ US: /sɪl/ ,(sil)

WordReference Random House Learner's Dictionary of American English © 2025

sill /sɪl/   n. [countable]

1. Building a flat, horizontal piece like a ledge beneath a window, door, or other opening.

WordReference Random House Unabridged Dictionary of American English © 2025

sill  (sil), n. 

1. Building a horizontal timber, block, or the like serving as a foundation of a wall, house, etc.
2. Building the horizontal piece or member beneath a window, door, or other opening. See diag. under double-hung. 
3. Geology a tabular body of intrusive igneous rock, ordinarily between beds of sedimentary rocks or layers of volcanic ejecta.

• bef. 900; Middle English sille, Old English syl, sylle; cognate with Low German süll, Old Norse syll; akin to German Schwelle sill

sill′-like′, adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

sill / sɪl/ noun

1. a shelf at the bottom of a window
2. a horizontal piece along the outside lower member of a window, that throws water clear of the wall below
3. the lower horizontal member of a window or door frame
4. a continuous horizontal member placed on top of a foundation wall in order to carry a timber framework
5. a flat usually horizontal mass of igneous rock, situated between two layers of older sedimentary rock, that was formed by an intrusion of magma

Etymology: Old English syll; related to Old Norse svill sill, Icelandic svoli tree trunk, Old High German swella sill, Latin solum ground