I'm learning spring cloud. I can see some definition about "microservice", but before that, I have not seen a exact definition about "service".
In the context of Spring Cloud, it is usually akin to microservice. The term does not refer to size, as that's a matter of your system architecture and does not really change anything from the point of view of Spring Cloud's libraries, but usually, the service will be an application in a microservice-based ecosystem that has a certain number of instances. The name or serviceId of that service is then used to find its instances, usually in Service Discovery.