The name of five different Roman Catholic popes, Sixtus is a Roman-sounding name made up by the Church to retroactively conceal the Greek roots of the first three popes to bear the name.
It turns out that a common name in the early Christian church was the Greek name Ξυστος, meaning "polished." Originally, this was rather accurately Latinized as "Xystus," and indeed, the first three popes named Sixtus are universally referred to as "Xystus" in early documents.
At some point however, somebody in the Church decided that this name did not fit in well with the clearly Latin names of the other popes, and from the early Middle Ages onward these popes were always referred to as "Sixtus" I, II, and III, apparently by analogy with the common Roman name Sextus, meaning "sixth." Thus, although Latin-sounding, "Sixtus" has no obvious meaning in Latin.
In later years, two other popes took the name "Sixtus," and used the faux Latin spelling from the get-go.
Notable people to have been named "Sixtus" include: