Halloween can be traced back to Samhain, the ancient Celtic harvest festival honoring the Lord of the Dead. Observed on November 1 in the British Isles and parts of France, Samhain also marked the beginning of the Celtic New Year. Because it was a time of transition between the old and the new, the Celts believed that the souls of those who had died during the previous year gathered to travel together to the land of the dead and it was also a time when those who had died before that returned to visit their homes. November 1 was also considered the end of the summer period, the date on which the herds were returned from pasture and land tenures were renewed. People lit bonfires to scare away evil spirits and "sacrificed" fruits and vegetables, hoping to appease the spirits of the deceased. Sometimes people disguised themselves in masks and costumes so that the visiting spirits would not recognize them. Charms, spells, and predictions of the future were all part of the eve of Samhain. In the old Celtic calendar, that last evening of October was "old-year's night," the night of all the witches.

When Christianity burgeoned, starting in the fourth century, pagan festivals like Samhain were very much frowned upon. However, the Celts would not give up their ancient rituals and symbols — so the Christian church gave them new names and meanings. November 1 became All Saints' Day (All Hallows' Day in England), by proclamation of Pope Boniface IV in the 7th century, a celebration of all the Christian saints. The evening before All Saints' Day, October 31, became a holy, or hallowed, eve and thus All Hallows' Eve (later Hallow-e'en, Hallowe'en, Halloween). Despite the name change, this holiday's association with the supernatural persisted.

Halloween came to be accepted in America with the influx of Irish immigrants in the 1840s. Their folk customs and beliefs merged with existing agricultural traditions. The early American Halloween, therefore, was not only a time to foretell the future and dabble in the occult, but was connected with seasonal tasks of the fall harvest. Over the years, the holiday's agricultural and harvest significance faded and it became primarily a children's holiday — one where they dressed up as the spirits (ghosts and goblins) that their ancestors at one time feared.

Bonfire comes from the words bone and fire ("fire of bones") and originally indicated a large open-air fire on which bones were burnt, either as a ceremony (like a funeral) or for burning heretics or banned books. The Halloween bonfires were lit to scare away evil spirits. Nowadays, bonfires are also celebratory — after a day at the beach or for a homecoming football game.

The word costume came to English via French from Italian for "fashion" or "custom, habit," from Latin consuetudo/consuetudinem meaning "custom." Mask also made a trip through French (masque) from Italian maschera/mascara, perhaps from Latin masca, "evil spirit, witch."

Ghost comes from an Old English word gast/gost, "spirit, soul" and has related forms in other West Germanic languages. These related words appear to be connected with Sanskrit hea, "anger, fury." Goblin is from French and it may be related to the German Kobold, a mythological spirit who haunted homes and lived underground in caves and mines. Etymologists believe it may be related to Greek kobalos and to Latin Gobelinus, mischievous spirits. The goblin carries the connotation of being grotesque and ugly, evil and malicious. The ghost is just downright scary, being the supposed soul of a dead person.

A jack-o'-lantern (also jack-a-lantern) is a hollowed-out pumpkin, originally a turnip, carved into a demonic face and lit with a candle inside. Light from a candle inserted inside can be seen flickering through the jack-o'-lantern's cutout eyes, nose, and usually grotesquely grinning mouth. The custom originated in the British Isles, with a large turnip or other vegetable rather than a pumpkin. The original meaning of the word jack-o'-lantern was "night watchman" or "man with a lantern," but it took on the Halloween sense by 1837, first in Nathaniel Hawthorne's Twice-Told Tales.

Pumpkin — the large fruit of the plant Cucurbita Pepo — is a word evolved from the original English spelling of pompeon or pumpion or pompion to pumkin and finally to pumpkin. The word pompion came from Latin pepo/peponem from Greek pepon, "large melon, edible gourd," from another word pepon, "cooked by the sun; ripe." Another spelling variant is punkin.

In Old English, witch was actually wicca and originally (c 890) was a man who practiced magic or sorcery, which we now call wizard. By the year 1000, witch came to be defined as "a female magician or sorceress."

The colors associated with Halloween are black and orange. Orange, the color of the jack-o'-lantern, is a symbol of strength and endurance as well as of autumn and the harvest. Black is primarily a symbol of death and darkness. The black of a witch's cloak and the black cat are reminders that Halloween was once a festival of the dead.