Backstab: Attacking someone you had previously been allied with.
Gunboat: A game where the players are not allowed to talk to each other.
Press: Talking.
Public Press: Talking in the 'global' tab in your game, which everyone can read.
Private Press: Talking with one country via the relevant country tab which only you and them can read.
Troll: Someone who is a nuisance on the forums.
Multi: Someone with more than one account on the site - this is NOT allowed.

CD: country disorder, an uncontrolled power
NMR: no moves received, when someone fails to set orders in time

AH: Austria-Hungary (the original boardgame name for Austria)
Anonymous: not knowing who the other players are
Meta/Metagaming: using information or influence outside the game to affect the game (lots of threads on this as some meta is OK, but other meta is frowned upon and has been declared a bannable offense by some of the mods).

Bounce/clash- two units moving into a third to create a collision and stop another unit moving there or too keep it empty so you can build there.

Anschluss: a German-Austrian alliance.

Yorkshire opening: English opening Edi>Nor, Lon>Nth, Lvp>Yor
Churchhill opening: English opening Edi>Nor, Lon>Nth, Lvp>Edi

The first is neutral and more defensive, the army at york can protect from a french fleet in the english channel.

The second is a very very pro-french opening that allows for basically every offensive option possible to England (except attacking france). You might get two builds with this one