sui generis

English contains many terms that ultimately trace back to the Latin forms gener- or "genus" (which are variously translated as "birth," "race," "kind," and "class"). Offspring of those roots include "general," "generate," "generous," "generic," "degenerate," and "gender." But "sui generis" is truly a one-of-a-kind "gener-" descendant that English speakers have used for singular things since the late 1600s. Its earliest uses were in scientific contexts, where it identified substances, principles, diseases, and even rocks that were unique or that seemed to be the only representative of their class or group. By the early 1900s, however, "sui generis" had expanded beyond solely scientific contexts, and it is now used more generally for anything that stands alone.