A flow of control within a single UNIX process address space. Solaris threads provide a light-weight form of concurrent task, allowing multiple threads of control in a common user-address space, with minimal scheduling and communication overhead. Threads share the same address space, file descriptors (when one thread opens a file, the other threads can read it), data structures, and operating system state. A thread has a program counter and a stack to keep track of local variables and return addresses. Threads interact through the use of shared data and thread synchronization operations. See also bound thread, light-weight processes, multithreading, unbound thread.