A term used in
politics to describe it when a member of a party votes in contradiction of his own party's
policy. The term is commonly used in
Britain and
Australia, both of which are based on the
Westminster form of government.
For example, if the Australian Labor Party's policy is to support homosexual rights, and a bill arises in parliament to implement this policy, and a member of parliament then decides to vote against the bill, this is called crossing the floor.
The term comes from the fact that this is exactly what happens within a house of parliament. To make things easier for counting, the government sits on one side of parliament, and the opposition on the other. To simplify counting, people move to different sides when votes are being cast. So, you actually do cross the floor to the other side of parliament when you cross the floor.
Crossing the floor is generally considered a treacherous act and political suicide, but there are exceptions, e.g. so-called conscience votes.