Ullage

Ullage \UHL-ij\ , noun;
1. The amount by which the contents fall short of filling a container, as a cask or bottle
2. The quantity of wine, liquor, or the like, remaining in a container that has lost part of its contents by evaporation, leakage, or use
3. In rocketry, the volume of a loaded tank of liquid propellant in excess of the volume of the propellant; the space provided for thermal expansion of the propellant and the accumulation of gases evolved from it

According to Etymonline.com:
This word, with the first definition, dates to the late 15th century and comes from an Anglo-French 14th century word ulliage. Ulliage comes from a 13th century Anglo-Latin word oliagium, which derives from Old French ouillage. Ouillage comes from ouiller ("to fill up (a barrel) to the bung"), which is literally "to fill to the eye," which makes sense when you know that "eye" in Old French is ueil. Ueil, by the way, comes from Latin ochulus.

It is so strange to think about how a word like this can start in Latin and end up being used to describe something related to rocket science. That's the beauty of language folks, it's malleability and ability to be recycled over and over again!

*Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Wednesday, November 10