Light Weight Kernel Threads (LWKT) is a computer science term and from DragonFly BSD in particular.
According to Matt Dillon, DragonFlyBSD creator: The LWKT scheduler is responsible for actually running a thread.
The mainframe z/OS Operating system supports a similar mechanism, called SRB (Service Request Block).
An SRB is similar to a Process Control Block (PCB), in that it identifies a unit of work to the system.
The Windows Operating System knows a similar light weight thread mechanism named "fibers".