A Portkey is an enchanted object which will transport a person to a pre-specified location. The object is usually a supposedly worthless piece of junk so as not to attract Muggle attention (GF6). There are two types of Portkey: the first is activated when touched, and the second only activates at a specific time (Pm).
An object can be transformed into a Portkey through use of the Portus spell (OP36). In Britain, creation of Portkeys is regulated by the Portkey Office of the Department of Magical Transportation in the Ministry of Magic (OP7, OP36).
Travel by Portkey is uncomfortable and can cause severe nausea. Healers recommend that the elderly, infirm and pregant should not travel in this fashion (Pm).
Before the introduction of the Hogwarts Express, some students would travel to Hogwarts via Portkey. This resulted in many of them having to spend the first few days of term in the Hospital Wing with Portkey Sickness (Pm).
200 Portkeys were used to transport spectators from strategic locations to the 1994 Quidditch World Cup. Timed Portkeys were used to stagger the arrival of witches and wizards, thereby avoiding security breaches (GF6).
Barty Crouch Jr. turned the Triwizard Cup into a Portkey which took Harry Potter and Cedric Diggory to the graveyard where Voldemort was reborn (GF32).
The Advance Guard used Portkeys to transport each of the pairs to Grimmauld Place after the Battle of the Seven Potters (DH5).
From the French 'porter' meaning to carry, and the English 'key' in the sense of trick or secret (Pm).