1. In anatomy and zoology, a small opening or perforation, as in a bone; especially, between the middle and inner ear; a window.
2. A transparent spot in an otherwise opaque surface, as in the wings of certain butterflies and moths.
3. Any opening resembling a window, as in some plaster casts, surgical drapes, or surgical instruments.
4. In architecture, a window-like opening; such as, in a wall.

The opening into the duodenum made by the common bile duct and the pancreatic duct.

A small, rounded, recessed opening in the medial wall of the tympanic cavity, located behind and below the fenestra vestibuli, under cover of the projecting edge of the promontory and communicating with scala tympani of the cochlea from which it is separated by the secondary tympanic membrane.

The two window-like, membrane-covered openings between the middle and the internal ear.

A membrane-covered opening between the middle and inner ears in amphibians and mammals through which energy is sent after traveling in the membranous labyrinth: The fenestra rotunda is a small opening; especially, either of two window-like apertures in the medial wall of the middle ear.

A vein of the internal ear draining the region of the round window, which drains into the vestibulocochlear vein.