REST architecture requires that each client request contain all the information needed for the server to process it, and that the server must not store any client-specific session context between separate requests. Which REST architectural constraint does this describe?

Statelessness is the REST constraint requiring that each request from a client to a server must contain all the information necessary for the server to understand and fulfil it, without relying on any stored context (session state) left over from a previous request. This means every request is handled in complete isolation, which improves scalability (any server instance can handle any request, since no server needs to "remember" prior client interactions) and reliability. Cacheability (whether responses can be cached), Uniform Interface (consistent resource-based interaction patterns), Layered System (intermediary layers like proxies/gateways), and Code on Demand (optionally sending executable code to clients) are all separate REST constraints.