Indeed I have just checked with Answers.Com and here is a slightly longer definition than mine:
Legal immunities conferred upon members of a legislature with regard to acts they may perform in the legislature or on its behalf. The principal parliamentary privilege in the UK Parliament is that of freedom of speech in its proceeding, given statutory expression in article nine of the 1689 Bill of Rights. This marked the parliamentary victory over the royal executive in the struggle that had lasted for most of the seventeenth century and ended with the flight of James II and Parliament's choice of William II to succeed him. No member may be held to account by an outside body or individual for words spoken within Parliament. Similar notions exist in most other democratic legislatures. Also surviving, but of diminished importance, are the privileges of freedom from arrest in civil process, freedom of access to the monarch, and rights of punishment against those abusing parliamentary privilege or those held to be in contempt of parliament.
— Jonathan Bradbury
Now there is no way scum like David Chaytor and his pals can hide behind that to avoid facing trial for alleged expenses offences. As people like New Labour politicians, and Tories and Lib Dems, love to say to the rest of us when imposing ever more draconian "anti-terror" legislation: "If you've nothing to hide you've nothing to fear". If I were an MP who had done nothing wrong I would want to face a court to prove my innocence.
I just about remember the olden days when politicians "did the honourable thing", even when there was only a perception of wrong doing, but it could damage the government, parliament or the country. Not any more. Not with the bunch of amoral, self-seeking, money grubbing, authoritarian, unprincipled scum we have stalking the corridors and lobbies of the Houses of Parliament at the moment.