Due to the increasing integration of renewable forms of generation, ageing network infrastructure, and rapid increase in peak load demand, flexibility is becoming economically more viable and hence significant role player in the future power system. There is vast amount of literature on flexibility covering research, demonstration and validation activities. Nevertheless, there is still no unifying definition of the term "flexibility" and consistent characterizing terms for "flexibility resources". The lack of clarity in definitions and concepts may undermine information exchange amongst stakeholders imposing hurdles on the transition from mature technology to investment decisions and deployment. System operators, for example, require better clarity for the techno-economic evaluation of flexibility resources in their planning processes. This paper, by reviewing prominent flexibility-related publications, proposes a comprehensive flexibility definition and unified characterizing terms for flexibility resources. Furthermore, the paper proposes a taxonomy method which is applied to classify flexibility resources. The presented taxonomy method clears the confusion on "what-is-what" under the concept of flexibility. This paper also presents the benefits of unified characterizing terms in mapping flexibility resources to ancillary services. The benefits are illustrated by considering a realistic use case in a Norwegian distribution network.