"It's often infested with Nargles."
-- Luna Lovegood(OP21)
Mistletoe is a plant, commonly used as a Christmas decoration, whose berries can be used as potion ingredients.
Ingredient in the Antidote to Most Poisons and the Forgetfulness Potion (Pm).
Mistletoe is used as Christmas decoration (PS12, CS12, GF23, OP21, HBP15), and was part of the decorations in the Room of Requirement just before the Christmas holidays. Luna remarked that Nargles frequently infest mistletoe. Later, Harry and Cho Chang were alone under the mistletoe, and this time Harry brought up Nargles out of nervousness, but Cho laughed and they kissed (OP21).
The Lovegoods have mistletoe growing on either side of their front door with a sign saying "PICK YOUR OWN MISTLETOE" (DH20).
Mistletoe berries are used in Antidote to Common Poisons and Forgetfulness Potion (Pm).
Old English "mistiltan" - Germanic "mist" = "dung" + tan "twig" for the way birds spread the seeds tree to tree
In the lore of the druids, mistletoe signified life-force and fertility, hence the tradition of kissing under it.