Threads are a program "thread of control", with several possible implementations. Time slicing is just one possible implementation. Explicit transfer of control is another (cooperative multithreading). Another is a separate processor for each thread/process. "Processes" are generally threads with different ownership/privileges, that usually can't be trusted to cooperate.
Simultaneous MultiThreading (SMT) or Hyperthreading cores have separate thread-local instances of some processor state (some sort of thread context at a minimum), but share other things (like functional units, load/store buffers, and maybe even renamed registers); the multiple threads per core are essentially time-slicing usage of those shared resources. Multiple cores will have less sharing, and have fewer resource conflicts (though they still usually share some level caches and memory/I/O interfaces). There have also been some fun (but ultimately less successful) computer implementations which time-sliced all resources to hide memory latency from each of many threads.
The Wikipedia pages
https://en.wikipedia.org/wiki/...
and
https://en.wikipedia.org/wiki/...
are pretty informative. Intel's "Hyperthreading" was a rebranding of SMT.
In recent years, hacks like Spectre have taken advantage of the resource sharing across threads in a processor to peek from one privilege domain to another, obtaining secrets they shouldn't have. Getting HT/SMT to work without allowing such hacks usually slows things down.
