There have been intensive research efforts in the last two decades or so to design and deploy electronic voting (e-voting) protocols and systems which allow voters and/or external auditors to check that the votes were counted correctly. This security property, which not least was motivated by numerous problems in even national elections, is called verifiability. It is meant to defend against voting devices and servers that have programming errors or are outright malicious. In order to properly evaluate and analyze e-voting protocols and systems w.r.t. verifiability, one fundamental challenge has been to formally capture the meaning of this security property. While the first formal definitions of verifiability were devised in the late 1980s already, new verifiability definitions are still being proposed. The definitions differ in various aspects, including the classes of protocols they capture and even their formulations of the very core of the meaning of verifiability. This is an unsatisfying state of affairs, leaving the research on the verifiability of e-voting protocols and systems in a fuzzy state. In this paper, we review all formal definitions of verifiability proposed in the literature and cast them in a framework proposed by Küsters, Truderung, and Vogt (the KTV framework), yielding a uniform treatment of verifiability. This enables us to provide a detailed comparison of the various definitions of verifiability from the literature. We thoroughly discuss advantages and disadvantages, and point to limitations and problems. Finally, from these discussions and based on the KTV framework, we distill a general definition of verifiability, which can be instantiated in various ways, and provide precise guidelines for its instantiation. The concepts for verifiability we develop should be widely applicable also beyond the framework used here. Altogether, our work offers a well-founded reference point for future research on the verifiability of e-voting systems.