Sentence:  A Conservative member of Parliament <WSD>crossed the aisle</WSD> this weekend to join the resurgent Labor Party of Tony Blair. 
Meaning of the word marked with <WSD> tags:  Of a member of a parliament, to resign from one's political party and join another party, resulting in moving from one's currently assigned desk or seat in the legislative chamber to a new desk or seat physically located with the other members of one's new party.