An alloy is a solid solution or a mixture that combines a type of metal and another element. For something to be an alloy, it contains at least one of the following: a mixture of metallic phases, solid solution of elements or an intermetallic compound in which no clear boundary exists between phases.
One common example of an alloy is gold, however, in nature, gold can be a pure metal. When someone buys jewelry, they are buying a gold alloy in most cases and the karat count of the gold expresses how much pure gold is present. Pure gold is 24 karats, but 10 karat gold is less than half pure gold. The remainder is other metals, such as silver.