Microservices
An architectural style where an application is a set of independently deployable services, each owning its data and aligned to a business capability.
The defining property is independent deployability. Small size, separate repositories and containers are consequences, not the definition — a "microservice" that must be released alongside three others is a distributed module, and it has the costs of distribution with none of the benefit.
What that property buys: teams releasing on their own cadence, components scaling independently, and failure isolation if it is designed for. What it costs: every in-process call becomes a network call that can fail slowly, transactions no longer span the boundary, and debugging needs tracing infrastructure you must build and run.
The honest positioning is that microservices are an organisational scaling technique that happens to be implemented in software. Below roughly four autonomous teams, the coordination problem they solve does not yet exist.