Cryptocurrencies are described as currencies, payment instruments, financial assets or commodities, depending on the institution and the discipline, and the literature has treated the technology and the economics of these assets largely apart. This conceptual article links the two through a single idea: cryptocurrencies are answers to the problem of keeping a shared record of ownership without a trusted keeper. We decompose institutional definitions and show that they diverge because each promotes a feature that matters to its own mission, and we define cryptocurrencies by three constitutive conditions: digital representation of value, cryptographic authentication of ownership, and validation by a protocol enforced by participants. We then argue that decentralised validation relocates trust instead of eliminating it, drawing on evidence about exchanges, mining and custody, and we map cryptocurrencies on two dimensions, the locus of trust and the anchor of value. The map positions payment cryptocurrencies, platform coins, tokens, stablecoins, permissioned projects and central bank digital currencies without overlap, and yields four propositions on risk, stability, regulation and investor behaviour. The framework offers economics, finance, computer science, law and Islamic finance a shared vocabulary and testable hypotheses.