Webster's English Dictionary

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sill

sill \'sil\ n [ME sille, fr. OE syll; akin to OHG swelli beam, threahold, 
   G]k selis crossbeam 1: a horizontal piece (as a timber) that forms the 
   lowest member or one of  the lowest members of a framework or supporting 
   structure : as 1a: the horizontal member at the base of a window  1b: the 
   timber or stone at the foot of a door : THRESHOLD  2: a tabular body of 
   igneous rock injected while molten between other rocks