A is for Algeron, Treaty of. Signed in good faith by the Federation. Only somewhat so by the Romulans.
B is for Battle of Cheron, which was either the climactic skirmish of the Earth-Romulan War in 2160 or the last conflict between the Romulans and the Federation that happened in 2311 and preceded the Empire's withdrawal into isolation for the next half-century.
C is for Commander. The rank held by the first Romulan we ever saw.
D is for D'Deridex-class Warbird...twice as long as a Galaxy-class Federation starship.
E is for Empire, they have one
F is for FIRE EVERYTHING! One of the last things Nero said before his death.
G is for Gal'gathong. Where there are spectacular firefalls.
H is for Hidden, which is what their cloaking devises allow their ships to be.
I is for Intelligent schemers.
J is for Jolan True. A common and traditional Romulan greeting.
K is for Khitomer Massacre. Which the Romulans perpetrated.
L is for Lovok, who was replaced by a Founder
M is for M'ret. A Vice-procouncil of the Romulan Empire who defected to the Federation.
N is for NCC-1701D, the registry designation of the Enterprise-D. [who faced off against Romulans on numerous occasions]
O is for Osol Twist, a Romulan snack
P is for Praetor, leader of the Star Empire.
Q is for Quasar, Murasaki. Which the Enterprise-D was studying shortly after confronting the Romulans in "Data's Day".
R is for Remans, their sister/slave race.
S is for Star Empire. The official name of their empire.
T is for Tal Shiar.