An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting metallic substance generally has properties significantly different from those of its components.
After Doctor Julian Bashir performed an autopsy on the Klingon first officer Hon-Tihl in 2369, he stated the cause of his death were the minor tritanium alloy fragments in his chest and upper arms and severe weapon burns. (DS9: "Dramatis Personae")
In 2373, the crew of the USS Voyager discovered that the supposed asteroid bombardment of a Nezu colony was in fact a deliberate attack when they found an alloy within an asteroid fragment. It was part of a guidance system used by the Etanian Order. (VOY: "Rise")