Hardware thread (logical processor)
Beginner
One job that a core keeps track of. A core with two hardware threads can juggle two jobs at once, so the computer sees it as two.
Novice
A set of program state (registers and program counter) that a core holds so it can run one instruction stream. A core with two hardware threads appears to the operating system as two processors. Intel calls them logical processors; RISC-V calls them harts.
Expert
The architectural state the hardware duplicates per context: registers, PC, some control state and interrupt controller. Everything else (pipeline, caches, predictors, execution units) is shared or partitioned among the core’s threads.
Explained in Multicore and vector units (Architectures).
See also: Simultaneous multithreading (SMT).