Cordon sanitaire derives from the French for quarantine line, and is usually a barrier that is implemented to stop a disease or something undesirable from spreading and gaining momentum. It can also refer to buffer states that are places around a nation that could be considered as a threat or pose an ideological threat.
In Belgium, the term stands for an agreement amongst mainstream parties to not cooperate at any government level with the far-right Flemish Block party.
See also