embrasure

Short definition from Wiktionary

Noun

1. 1
   Any of the indentations between the merlons of a battlement; an opening in a wall or parapet through which ordnance can be fired.
2. 2
   Any small protected space.
3. 3
   The slanting indentation in a wall for a door or window, such that the space is larger on the inside than the outside.
4. 4
   An embrace.