Yajna is a large-scale fire sacrifice in which numerous priests recite Vedic hymns.
The smaller version is Homa or Havan.
The term yajna can be used for all devotional offerings, services, rites. To recite a mantra (Japa) is a Japa Yajna, to pass on knowledge/wisdom (Jnana) is a Jnana Yajna.
The term is transliterated manifold.