scupper

A scupper is an opening in the side walls of a vessel or an open-air structure, which allows water to drain instead of pooling within the bulwark or gunwales of a vessel, or within the curbing or walls of a building. 

There are two main kinds of scuppers:

1. Ships have scuppers at deck level, to allow for ocean or rainwater drain-off.
2. Buildings with railed rooftops may have scuppers to let rainwater drain instead of pooling within the railing. Scuppers can also be placed in a parapet, for the same purpose.