An ounce is a standard unit of the volume used in the United States & United Kingdom, which is symbolised by a symbol of “Oz”. It is commonly used to measure the liquid Quantities also known as Fluid Ounce.
There are also different types of the ounces very well known as a troy ounce, tower ounce, avoirdupois ounce. They have used by United States customary unit system and United Kingdom Imperial system of measurements.
Ounce is a British derived customary system and was primarily used for measuring of food portions, postal items, areal density of fabric and paper, boxing gloves, and so on.
An ounce is basically a measuring unit for weight as well as liquid. We often refer ounce as a small part of something but mathematically one ounce is equivalent to 28.3495 gram.
1 ounce = 28.3495 gram
Note: this is the United states customary and British imperials ounce
And for weighted measurements we apply ounces abbreviated as Oz. For liquid, fluid ounce is applied and is abbreviated as Fl oz
For fluids
1 US fluid ounce = 29.5735 mL
1 imperial (UK) fluid ounce = 28.4131 mL
Even after the rise in the popularity of the metric system ounce is still a standard unit in the United States. In the United Kingdom it ceased to be a legal unit of measure in 2000, but is still in general usage on an informal basis. In addition, it is the normal measure for portion sizes in British restaurants.
An ounce is a unit of mass in the system of English measurements used mainly in Europe, America, Russia, and other western countries.
It is, in fact, the name of several units of measurement of mass, also the two measures of the volume of the liquid body, a unit of measurement of force and several monetary units and, formed as the twelfth proportion of other units.
It comes from the ancient Rome, where an ounce as designated the twelfth part of the libra.
It was one of the main weight units of medieval Europe and even today, it is used in many counties where the weight is measured in pounds.
It is widely used in the banking, pharmaceutical, and jewellery industry.
In ancient Rome, 1 / 12 of the unit weight were called the libra. An ounce was marked by a dot ( · ) or ( on coins ) in the form of a small convex hemisphere , sometimes a horizontal line ( – ), and also( in cursive ) by the signs and; The signs Σ , Є , ξ , sicilica – Ɔ , sextula – Ƨ , scrupula – E served as ½ ounces ( semuncia ).

 Measures the length (ounce = 1 / 12 Roman ft = 0, 0246 m = 9 685 lines),

 Measures of surface ( ounce = 1 / 12 uger = 2400 Roman square feet = 46 square blacks ., 6 sq . ft ., 72 square inch = 209 , 91 m ²),

 Measures of capacitance ( oz = Ciatim = 1 / 12 sekstariya = 0 , 0372 circles ) and

 Cash marks (ounce = 1 / 12 ace) – The unian division was applied by the Romans in fractional computations. Decimal fractions of first place after the decimal point occupy tenth share, second – hundredths, and so on.

 The Romans expressed in fractions of a series of quantities, in which the denominator was a multiple of 12, and the first place was occupied by ounces, the second is the semionia.
The Use of an Ounce before the Introduction of the Metric System of Measures:
It is a wellknown fact that ounce was borrowed from the Romans by almost all European nations.
Before the introduction of the metric system of measures, it was the most common unit of weight in the world.
In Germany, an ounce was equal to 1 / 16 of a large shopping pound (= 1/8 marks) and 1 / 12 of small drugstore weight. In 1872, it was replaced by the metric system.
Ounce and Metric System of Measurements:
Ounce  Abbreviation  Brief Description 
Ounce Avoirdupois  oz or oz at 
In America and a number of other countries, the basic unit of weight is pound Avoirdupois, which consists of 16 ounces. It should be noted that of Avoirdupois is 28, 35 g. 
Liquid Ounce  fl oz 
It measures the volume of liquid. There are English and American liquid ounces. American ounce for liquid measurement is 1 / 16 pints or 29,573 531 ml. In the British imperial system of measures, 1 imperial pint consists of 20 ounces. In the USA, only for labelling food products, a liquid ounce is equivalent to 30 ml. 
Troy Ounce  t oz or ozt 
It is equal to 31, 1,034,768 grams. It is mainly used in the banking and jewellery business for measuring the weight of the precious metal. The term comes from the Troy weight system, which was widely used in Europe right up to the introduction of the metric system of measures. 
Apothecary Ounce (It is no longer used).  – 
It is not used now. In several countries, it was used to weighing medicinal substances. The Standards of pharmacy weight varied in different countries. 