Origins
The word 'ounce' enters English through Old French 'unce,' from Latin 'Ε«ncia,' meaning 'a twelfth part.' The Latin word derives from 'Ε«nus' (one), ultimately from PIE '*Γ³ynos' (one).βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ In the Roman duodecimal system, the 'Ε«ncia' was the fundamental fraction: one-twelfth of a 'lΔ«bra' (pound) in weight and one-twelfth of a 'pΔs' (foot) in length. This double function produced two English words: 'ounce' (the weight, via Old French) and 'inch' (the length, via Old English 'ynce'). They are doublets β two words from the same source that entered the language by separate routes and now look and sound entirely different.
The abbreviation 'oz.' does not come from the English word. It derives from medieval Italian 'onza' (from the same Latin 'Ε«ncia'), and was adopted into English commercial writing as a convenient shorthand. This is similar to 'lb.' for pound (from Latin 'lΔ«bra') β English measurement abbreviations often preserve Latin or Italian forms rather than English ones.
The ounce has always had multiple values, a complication inherited from the medieval European system of different pounds for different commodities. The avoirdupois ounce, used for most goods, is one-sixteenth of an avoirdupois pound, approximately 28.35 grams. The troy ounce, used for precious metals and gemstones, is one-twelfth of a troy pound, approximately 31.1 grams. The troy system takes its name from Troyes in Champagne, France, a major medieval trading center whose weight standards were widely adopted for high-value goods. The apothecaries' ounce, used in pharmacy, was identical to the troy ounce but divided into different sub-units (scruples and drams rather than pennyweights).
Development
The existence of multiple 'ounces' has caused centuries of confusion. Counterintuitively, a troy ounce is heavier than an avoirdupois ounce (31.1g vs. 28.35g), but a troy pound is lighter than an avoirdupois pound (373.2g vs. 453.6g) because the troy pound contains only 12 ounces while the avoirdupois pound contains 16. When gold is quoted at a price 'per ounce,' this always means the troy ounce.
The fluid ounce β a unit of volume, not weight β was originally defined as the volume of one ounce of water, exploiting the convenient near-equivalence of weight and volume for water. The US fluid ounce (29.57 ml) and the Imperial fluid ounce (28.41 ml) differ slightly, adding yet another layer of transatlantic confusion.
Note: the 'ounce' that names the snow leopard (Panthera uncia) has a completely separate etymology. It comes from Old French 'once,' a misanalysis of 'lonce' (the lynx) β the initial 'l' was mistaken for the French article. That 'lonce' descends from Latin 'lynx,' from Greek 'lynx.' The two 'ounces' are false friends: one measures weight, the other stalks the Himalayas.