An alloy is a mixture of two or more metals.
Jewellers often refer to the mixture of metals added to the fine metal as “the alloy”, though technically the entire mix is an alloy.
Precious metals are alloyed for various reasons: to change the colour of the metal (e.g. white gold or rose gold), to change the working properties of the metal (e.g. some alloys are better suited for casting processes, stamping or hand manufacture) or to lower the cost of the metal (e.g. 9ct gold has less fine gold in the alloy than 18ct gold and will therefore be less expensive).