ullage

ull•age (UHL-ij)  n

Definition:
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. 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

ullaged adjective

Origin:

The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was created, to fill a barrel up to the bung hole. In turn, a noun ouillage was created, which was the immediate source of our word, first recorded in Norman English about 1300, at first in the sense of the amount of liquid needed to fill a barrel up to the bung hole.
By an obvious extension, ullage came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. And it is also applied to the unfilled air space at the top of a bottle of wine, which in this case is essential to allow for expansion of the contents as the temperature changes.

Related:
Related Words: oeillade “an oogling stare, an amorous gaze”

Sentence Examples:
• In the lowest cellars reserved wine in cask is stored, as it best retains its natural freshness and purity in a very cool place. All air is carefully excluded from the casks, any ullage is immediately checked, and as evaporation is continually going on the casks are examined every fortnight, when any deficiency is at once replenished.

• "At all events, I'm not ashamed to look mine enemy in the face—so hand us out the bottle."
Moonshine put the bottle on the table.
"Now, Bob," said Cockle, "what d'ye say to a seven bell-er? Why, hallo! what's become of all the grog?"
"All drank last night, Massa Cockle," replied Moonshine.
"Now, you ebony thief, I'll swear that there was half a bottle left when I took my last glass; for I held the bottle up to the candle to ascertain the ullage."
-Olla Podrida, Frederick Marryat

The Storyline
This was definitely shaping up to be one of those days that is more ullage than wine.

Why This Word:
We may not think much about the empty space in a bottle or container for liquids, but judging from my search results, engineers sure do.
I like this word for two reasons. Firstly, it's an interesting example of language change. Ullage went from the hole, the "eye", in cask, to the amount of wine it took to fill the cask to the hole, to the amount of space left in the cask above the hole, to the amount generally of empty space in a closed vessel for liquids. Thus the word for the empty space in a wine bottle is related to the words monocle and binocular. See?
Secondly, like kerf, another word for what isn't there, and limen, a word for the space in between, it's a word ripe with untapped metaphorical meaning. Ullage is empty space, but it's purposeful empty space.

Sources: Dictionary.com, Wikipedia

Word-E: A Word-A-Day