Now maybe not all of your are as au fait with the concept of swing as David Butler or Peter Snow so here it is in a nutshell.  It is the difference between the percentage of a party’s vote in one election compared to the previous election.  It is a little bit more complicated than that.  The best way to explain it is using an example from a Parliament research paper. 

The largest national swing from one party to another was in the 1945 general election.  There was an average swing of 12 percent from the conservatives to labour.  This great labour landslide resulted in a majority of 145 seats.