WordNet (r) 3.0 (2006):

Hallel
    n 1: (Judaism) a chant of praise (Psalms 113 through 118) used
         at Passover and Shabuoth and Sukkoth and Hanukkah and Rosh
         Hodesh

Easton's 1897 Bible Dictionary:

Hallel
   praise, the name given to the group of Psalms 113-118, which are
   preeminently psalms of praise. It is called "The Egyptian
   Hallel," because it was chanted in the temple whilst the
   Passover lambs were being slain. It was chanted also on other
   festival occasions, as at Pentecost, the feast of Tabernacles,
   and the feast of Dedication. The Levites, standing before the
   altar, chanted it verse by verse, the people responding by
   repeating the verses or by intoned hallelujahs. It was also
   chanted in private families at the feast of Passover. This was
   probably the hymn which our Saviour and his disciples sung at
   the conclusion of the Passover supper kept by them in the upper
   room at Jerusalem (Matt. 26:30; Mark 14:26).

     There is also another group called "The Great Hallel,"
   comprehending Psalms 118-136, which was recited on the first
   evening at the Passover supper and on occasions of great joy.