The Yulai Convention is an RP term for a number of game mechanics and systems within eve.
It is primarily seen in war CONCORD mails when an the attacking party leaves a bill unpaid, leading to a mail stating that "CONCORD has declared this war invalid as it breaches one or more articles in the Yulai Convention".
The convention also covers the definitions of time used by the four major empires, meaning that all time systems in New Eden run on the Gregorian calendar, using the UTC clock.