A fuel reserve was an allotment of fuel to be used in emergency situations. In the 23rd century, a fuel reserve was included aboard a Romulan Bird-of-Prey that served as the flagship of the Romulan Praetor.
During the Neutral Zone Incursion of 2266, the fuel reserve aboard the aforementioned Bird-of-Prey was entirely depleted. The ship's commander cited this as a reason for not destroying the USS Enterprise, when a subordinate of his, Decius, pressured him to do so. (TOS: "Balance of Terror")