How does a materialized view differ from an ordinary CQRS projection?

An ordinary CQRS projection keeps one service's own read model in sync with events published by that same service's own aggregate — an account service's balance view listening only to its own account events, for example. A materialized view deliberately combines events published by several different aggregates across several different microservices into one unified record. The distinguishing feature isn't the mechanism, since both use event handlers to update a read model, it's the scope: a projection answers a question one service can answer alone, while a materialized view answers a question that genuinely spans service boundaries, such as a customer profile screen needing data from customer, account, card, and loan services at once.