A mixture of two or more pure metals. In the case of gold, a combination with one or more metals, often base metals such as copper, zinc and silver, to lower the purity, influence the colour or add hardness and strength. Initially from the Latin alligare, to bind up.

Alloys of gold are used both for coins and jewellery and are usually made by semi-fabricators for the jewellery trade, although some jewellery manufacturers make their own alloys.
The most common alloys are 22, 18, 14, 9 and 8 carat, but may be made to other standards in some countries, such as 19.2 carat in Portugal.

The colour of the alloy depends on the balance of other metals with which gold is mixed. In coloured gold’s, red shades are achieved by rising the copper content at the expense of silver and zinc; fair yellow and green shades are achieved by increasing the silver and/or zinc content at the expense of the copper.

At a given gold level, copper-rich gold’s tend to be harder than silver-rich gold’s. White gold’s are achieved by alloying with nickel or palladium, often with some zinc and copper to improve flexibility. Nickel has a better hardening effect than palladium.

An alloy is a mixture, either in solution or compound, of two or more elements, at least one of which is a metal, and where the consequential material has metallic properties. An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The resulting metallic substance generally has properties considerably different from those of its components.

Alloys are usually designed to have properties that are more attractive than those of their components. For instance, steel is stronger than iron, one of its main elements.

Unlike pure metals, most alloys do not have a single melting point. As an alternative, they have a melting range in which the material is a combination of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. However, for most pairs of elements, there is a specific ratio which has a single melting point, and this is called a eutectic mixture.

In practice, some alloys are used so principally with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 carat (58%) gold is an alloy of gold with other elements. Similarly, the silver used in jewellery and the aluminium used as a structural building material are also alloys.
The term "alloy" is sometimes used in everyday speech as a synonym for "aluminium alloy"; one example of such usage is the "alloy wheels" which might be fitted to a car. (This usage is clearly imprecise, since all steels and most other metals in practical use are also alloys.)