The English situation

The terminology for the parties to civil litigation in England has recently changed, so one now needs to understand the historical terms that have been used since the use of modern English, and the new terms. The terms "prosecution" and "defendant" have been retained for criminal litigation.

"The suer"

New style
"claimant"
Old Style
"plaintiff"
"Suing" or "bringing suit" is now known as "Claiming", or appropriate variants thereof.

"The sued"

New style
"respondant"
Old Style
"defendant"

"The appealer"

This is still the "appellant".

"The appealed against"

This is still the "respondant".

Counter-claims

In the High Court, where the respondant counter-claims against the claimant on a related matter, the two claims may be joined and heard as part of one case, with only one case number issued, and one court process: In these cases, whoever brought claim first is the claimant (not that it matters very much in such cases). This tends to happen in patent infringement cases, where the respondant often counter-claims for the patent to be struck down.