In recent years, the issue of flexibility has become increasingly important to power system planning. Generation flexibility has been under continuous research for many years; however there is little research on transmission expansion planning flexibility, which has attracted more and more attention due to the increasing uncertainties in a competitive electricity market environment. Planners have to carefully consider the resulting costs, benefits and risks during the planning phase. Future electricity demand is a main source of uncertainty facing transmission planners. Flexibility is a useful way to deal with it. The most flexible plan can reduce the probability of loss of demand, cut down the cost of reconfiguration and save reconstructing time. In this paper, we classify the transmission flexibility into three dimensions including market flexibility, operational flexibility and expansion flexibility with detailed definitions. Transmission expansion flexibility (TEF) is the main focus of this paper and can be used as an important criterion for decision making and transmission planning. Case studies based on two test systems are used to illustrate the proposed TEF and its application in power system planning. The case study results clearly demonstrate the effectiveness of the proposed method in transmission expansion planning. The use of TEF can also help transmission system investors to achieve more returns from the competitive market