A GC can either cause a stop-the-world (STW) situation, or objects can be collected concurrently without stopping the application. The GC algorithm can be executed in a single thread or in multithread. So, Concurrent GC does not mean it executes in parallel, whereas Serial GC doesn't mean it causes more pauses due to serial execution. Concurrent and Parallel are different, where Concurrent means the GC cycle, and Parallel means the GC algorithm.