Integration test is something that tests a functionality that is dependant on a external system e.g. a database, HTTP API or message queue.
Integration vs unit tests
The line is thin in my opinion. The integration part can be faked or a embedded services can be used in place of the actual integration point and with these solutions the interaction with the external system is bounded in the test context and the tests can be executed in isolation so they are very much like unit tests. The only difference with this type of integration test and unit test is that the startup time of the embedded or faked system usually takes some seconds and that adds total execution time of the tests. Even though the total test exection time is longer all the tests need to pass and all the cases need to be covered whether there's external systems involved or not so the importance is equal between the test types.
This is why I wouldn't separate unit and integration tests from each other within the code base but treat them as equal.
If there's a need to run a smaller set of tests while developing I'm quite sure all test libraries or IDE's support running tests by giving some type of filter so that the whole test suite doesn't need to be run every time a change is made if that's the goal of the separation.
Fakes are a great way to test the logic of the application without actually interacting with a external system. A good example of a fake implementation is a memory based hashmap that can be a good enough fake implementation of a database.
Embedded services i.e. services that are started in the test code and stopped by the test code are a good option when testing integration of systems with automated tests but they don't neccessarily provide all the functionalities and capabilities of a real system so there's usually always the edge cases where embedded services are not viable.
Containers what most of use recognize as docker containers also provide a good solution for running tests that need to interact with external services. In my opinion this is probably the best way to tackle this issue though it's not as fast as fakes and embedded services and it does make your tests dependant on the containers but I think it's worth the extra running time and dependency.
In the next part I'll be writing of another test aspect i.e. acceptance testing.