a window with a sash that slides into a space below the sill.
Origin of drop window
First recorded in 1900–05
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for drop window
A shelf or counter can be fitted inside the drop-window for the display of your wares.Woodworking for Beginners|Charles Gardner Wheeler