A one-hit wonder is a composer who wrote one work that is much more popular than the rest of their music. In many cases the other works are rarely, if ever, performed today, although they might have had some popularity when originally written.
There are numerous examples of one-hit wonders in both classical and popular music. One of the most often cited of the former is Pietro Mascagni. His 1890, one-act opera "Cavalleria rusticana" is a true masterpiece and is frequently performed as part of the standard repertory. However, his other works, including fifteen operas and several orchestral and piano pieces, have fallen into obscurity and are rarely heard today.