"You has to eat this, sir! ... Right before you go into the lake, sir - Gillyweed! ... It will make Harry Potter breathe underwater, sir!"
-- Dobby (GF26)
Gillyweed is a water plant native to the Mediterranean which looks like a bundle of slimy, greyish-green rat tails, and has magical properties that allow its consumer to breathe underwater.
When eaten, it gives a person gills to breathe underwater and webbed hands and feet for swimming. It also causes other fish-like qualities such as lack of blinking and low water temperature adaptation. The duration of the effect is approximately one hour (GF26). If Gillyweed is eaten above water, the sensation of the effect feels "as though an invisible pillow [is] clapped over [your] nose and mouth". It is very rubbery and difficult to chew (GF26).
- Snape keeps Gillyweed in his private stores; it is not available to the students. Snape believed Harry had stolen the Gillyweed he used in the second task of the Triwizard Tournament, but it was actually Dobby (GF26, GF27).
- Barty Crouch Jr. as fake Moody tried to give Harry a hint about Gillyweed by giving Neville Longbottom the book Magical Water Plants of the Mediterranean. When that didn't work, he let Dobby overhear a "loud conversation" about it with McGonagall in the staff room, who then stole some from Snape's stores and took it to Harry (GF35).
- The effects of Gillyweed were first discovered by Elladora Ketteridge; however, Herbologist Beaumont Marjoribanks was given official credit for the discovery about a century later (FW).
- In one of the possible timelines following the use of the Experimental Time Turner, Albus Potter and Scorpius Malfoy take Gillyweed when they re-visit the second task of the Triwizard Tournament (CC2.19).
"Gill" = a breathing organ found in many aquatic organisms
In the film, the book Crouch Jr gives to Neville is called "Magical Water Plants of the Highland Lochs" and Neville, not Dobby, gives Harry the Gillyweed (GF/f).