"But unless you watch your step, you might just find that my hand slips ...right over your evening pumpkin juice. And then Potter ... then we'll find out whether you've been in my office or not."
-- Snape threatening to spike the pumpkin juice with Veritaserum (GF27)
"Tea? Coffee? Pumpkin juice?"
-- Dolores Umbridge, trying to trick Harry into taking Veritaserum (OP28)
Pumpkins are a squash, a vegetable used for food, drink, and ornamental purposes.
Although the plants aren’t inherently magical, Hagrid gives them some “help” so that the pumpkins swell to the size of garden sheds by the time the feast rolls around (CS7, CS8). He is quick to rescue them when Ron vomits some slugs nearby (CS7), but was unable to stop the blast-ended skrewts, who “rampaged” in the patch and withered the vines (GF21).
Representing pumpkin-growers, Dagbert Pips, the owner of the Pumpkins R Us, protested against planned Ministry of Magic restrictions on Hallowe’en celebrations. Sales of his specialty “luminous pumpkins, flying pumpkins, singing pumpkins and exploding pumpkins” would be hit hard by the new regulations (DP4).
For the Hallowe’en feast in fourth year, “hundreds of carved pumpkins leered from every corner” (GF16).
Pumpkin juice is a staple drink around the Hogwarts table (CS5, PA6, GF12, OP11) and students drink it outdoors on sunny days (PA16). Fred and George Weasley also obtain pumpkin juice and fizz directly from the kitchens for impromptu parties (CS10, PA13).
Snape threatened once to put a drop of Veritaserum in Harry’s pumpkin juice to force the truth out of him (GF27). Dolores Umbridge actually tried to do the same when she attempted to find out from Harry where Albus Dumbledore was (OP28).
Pumpkin juice does not seem to be connected with potions.
From the Greek pepon (πέπων) “large melon"
On Halloween in the United States, pumpkins are traditionally carved into "jack-o-lanterns" with a face lit up by a candle. But in England and Ireland the traditional vegetable used on All Hallow's Eve was the turnip or the related swede (rutabaga). In literature, Washington Irving told the story of the Headless Horseman who chased Ichabod Crane away from Sleepy Hollow by throwing a carved pumpkin at him. In L. Frank Baum's Wizard of Oz books, there is a character named "Jack Pumpkinhead, and in Tim Burton's film "The Nightmare Before Christmas," the "Pumpkin King" is named Jack Skellington. And of course the most famous of all is Charles Schultz's holiday creation "It's the Great Pumpkin, Charlie Brown" with Linus Van Pelt and Lucy Brown missing tricks or treats, spending Halloween in the "most sincere" pumpkin patch (Wikipedia).
Pumpkins are really a fruit rather than a vegetable (Wikipedia).