Juggernaut: Russia/Turkey alliance
Western Triple (or WT): England/France/Germany alliance.
Sealion: A specific plan to attack England by Russia, France, and Germany. Sometimes used colloquially to refer to a less specific plan being pitched by two of those powers.
Lepanto: A series of Italian and Austrian moves designed to get Italy into Turkey
Key Lepanto: The stupidest thing Austria can do, ever.
Stalemate line: A place where you can stop powers on one side of it from getting to the other side, used to force a draw. The most common one is bordered on the west by St. Petersburg, Livonia, Prussia, Silesia, Munich, Switzerland, Marseilles, Spain, and Mid Atlantic Ocean.
MAO: Mid-Atlantic Ocean
Oftentimes, powers are referred to by their initial, especially if someone is trying to tell you about an alliance or offer an alliance. For instance, EG would be England/Germany.