The gill is a unit of volume in the imperial unit system and uses the symbol gi.
It represents 1/4 of a pint - so in this instance, it is 1/4 of an imperial pint - therefore measures exactly 142.0653125 ml. It's also equal to 2 imperial cups and 1/32 of an imperial gallon.
It is pronounced with a "J" sound at the beginning - and it actually part of a humorous in the nursery rhyme 'Jack & Jill'.
In the UK it was used throughout pubs as standard measures; for example spirits were served as 1/6 gill as standard. In Scotland it was 1/5 gill and in Ireland 1/4 gill. It's still used in the Republic of Ireland today.
The acre-foot is a unit of volume in the US customary unit system with the symbol ac⋅ft.
It represents the volume contained in a box measuring 660 feet long, 66 feet wide and 1 foot deep. This can be thought of a box the size of an acre but 1 foot deep.
It is approximately the same volume as an 8 lane swimming pool; 25 metres in length, 3 metres deep and 16 metres wide (assuming lanes are 2 metres wide).
It is still used in the US despite its links to the much-outdated imperial system of units. For example a "rule of thumb" is that an average suburban family's annual water usage should be around 1 acre-foot. This is equal to just under 3.4 m³ daily.