Two questions for British posters

[Linus Van Pelt]

Enjoying your various detailed election threads, but you've got some crazy lingo there.

1. What are "notionals"?

2. What is the "swing", such that, as seems to be the case, the "swing" is from just one party to just one other party, even though there are three significant parties (or more in some areas)?

[Linus Van Pelt]

Thanks!