'If you proceed downstairs into the Department of Mysteries, Cornelius ...you will find several escaped Death Eaters contained in the Death Chamber, bound by an Anti-Disapparation Jinx and awaiting your decision as to what to do with them.'
-- Dumbledore (OP36)
The Anti-Disapparation Jink prevents someone from Disapparating, or disappearing to magically travel somewhere else.
References from the canon
- Albus Dumbledore captured the Death Eaters who had been fighting in the Department of Mysteries and held them with an Anti-Disapparition Jinx (OP36).
- Most wizarding dwellings are magically protected against unwanted Apparators, according to Albus Dumbledore, who confirmed that "you can't Apparate anywhere inside the buildings or grounds" of Hogwarts (HBP4). However, the Headmaster or Headmistress can temporarily lift the restriction from a specific area of the school for short periods, so that someone already within that area can Apparate to another place within the same area, although they cannot leave the area by Apparition (HBP18).
While not stated as such in canon, it seems a reasonable deduction that the Prime Minister's office is protected against Apparition as an elementary security measure, since Fudge's visits to the Prime Minister there have always involved the Floo Network rather than Apparition, and since protection against Apparition isn't mentioned as part of the Prime Minister's new security arrangements (HBP1).
From the Web
"Anti-Disapparition Jinx" on Harry Potter Wiki