The term "API gateway" in microservices architecture refers to a server that acts as a single entry point for multiple microservices. It aggregates requests from clients and routes them to the appropriate service, managing various functions such as authentication, load balancing, and request logging. By centralizing these responsibilities, an API gateway simplifies client interactions and enhances performance. It also provides a layer of security by protecting the underlying services from direct exposure, ensuring a more manageable infrastructure for developers and improved user experiences.