[Thread of execution and the blocking thread] The term "thread", like "process", is most commonly associated with the operating system level, but also exists above it at the user or application level as either green or lightweight threads. System or kernel threads are relatively heavyweight in that allocating memory for the stack limits the possible concurrency, and context switching between system threads using system calls adds significant overhead; conversely, threads at a user level are more lightweight in both aspects and thus can offer better scalability for certain use cases.

The difference between green and lightweight threads is that green threads aren't necessarily lightweight; an example of green threads is classic Java Virtual Machine threads, while Go goroutines are an example of lightweight threads.