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 imperial pint is a unit of volume in the imperial unit system with the symbol pt.
1 imperial pint is equal to 1/8 of an imperial gallon. This is equal to 1/2 imperial quart, 4 imperial gills or 20 imperial fluid ounces. In SI / metric this equates to 568.26125 ml.
In Britain, all beer, ales and stout must be served in pubs by the pint (or half pint). Every single glass, despite being a 'pint' must have the numbers 568 ml printed on the side - and because of this, is likely the most commonly known conversion factor amongst the British population.