Sash refers to the part of the window that secures the glass in the main window frame while still allowing the glass to be repositioned so that the window can be opened.
For example, in a double hung window there is typically one main window frame which attaches to the wall to secure the window to the building and two smaller frames (the sashes) which hold the glass.
One sash is positioned in the lower half of the window while the other sash is positioned in the top half.
The bottom sash can then be slid upward or the top sash may be slid downward to open the window.
Depending on the window style, a sash may be opened in other ways as well such as sliding horizontally, swinging outward like a door, or tilting upward like an awning.
