Tantum Ergo Sacramentum, while usually treated as a hymn of its own, is actually the final two verses of the Pange Lingua Gloriosi, a Eucharistic hymn that was composed by St. Thomas Aquinas for the Feast of Corpus Christi.
The Tantum Ergo is most commonly sung today at exposition and benediction, when the Blessed Sacrament is exposed for adoration. This is a traditional English translation of the Latin hymn.
Tantum Ergo Sacramentum
Down in adoration falling,
Lo! the sacred Host we hail;
Lo! o'er ancient forms departing,
newer rites of grace prevail;
faith for all defects supplying,
where the feeble senses fail.
To the everlasting Father,
and the Son who reigns on high,
with the Holy Ghost proceeding
forth from Each eternally,
be salvation, honor, blessing,
might and endless majesty. Amen.