This is an intriguing question, as chant encompasses such a stunningly wide array of musical expression. From the Latin word cantare, meaning "to sing," chant can weave beautiful melodies that send the heart soaring, as we hear in some of the recent recordings of Celtic chanting. Some chant melodies have been carefully preserved over centuries, perhaps painstakingly transcribed by quill pen on parchment by a tonsured monk, and are always sung in precisely the same tonal sequence.
In contrast, the Orthodox Jewish davennen [the traditional Jewish form of chanted prayer], while adhering to specific conventions regarding melodies, is somewhat more improvisational, treating the traditional melodies like jazz "riffs," altering rhythms to fit the changing text into the melody, and subtly adding musical embellishment. Relying on traditional melody completely gives way to the creative Spirit, as Pentecostals bring forth spontaneous outpourings of sacred sound in their practice of speaking in tongues. Other forms eschew melody altogether. For example, Tibetan Buddhist monastic chant often drones on only one fundamental tone; or in the case of some Sufi zhikrs, the repetitive text is chanted on a half-spoken tone of indistinct pitch.
Much chant is rhythmic, from the pounding heartbeat of Native American drums, to the polyrhythmic chants of West Africa, to the incredibly complex rhythmic patterns of the Balinese Monkey Chant. On the other extreme, chant may consist of long sustained notes with no rhythm at all, as in the traditional toning of the well known Sanskrit sacred syllable "OM."
Chants serve many purposes: telling stories such as the mythic tale of Lord Rama and Sita in the great Indian epic the Ramayama, casting out disease in the healing chants of the Siberian shaman, instructing young family members in the proper patterns for fine Kashmiri carpets, or inducing trance in Haitian voodoun. Chant is used to quiet the mind, open the heart, uplift the spirit, and mourn the dead.
Having tasted the incredible richness of the world of chant, we see that dictionary definitions are either incorrect--"a simple liturgical song in which a string of syllables or words is sung to each tone," or absurd--"any monotonous song." For our purposes, let us define chant as: the worship and celebration of the sacred through melodically simple vocalization.
Chant is singing our prayers. Chant is vocal meditation. Chant is the breath made audible in tone. Chant is "discovering Spirit in sound."