I want to clarify whether "a schedulable unit of CPU time slice" is "process" or "thread" (kernel managed thread). What I mean by "schedulable unit of CPU time slice" is the unit which CPU scheduler of an operating system allocates CPU time slice.
According to "Short-term scheduling" in wikipedia, process is used to refer the schedulable unit.
"This scheduler can be preemptive, implying that it is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process"
Also, according to "Time slice" in wikepedia,
"The scheduler is run once every time slice to choose the next process to run."
Also, according to "Thread" in wikepedia,
"a process is a unit of resources, while a thread is a unit of scheduling and execution"
According to "Processes and Threads" in microsoft docs,
"A thread is the basic unit to which the operating system allocates processor time."
According to "Is thread scheduling done by the CPU, kernel, or both?" in quora,
"The CPU (hardware) just carries out instructions. The CPU itself has no concept of threads or scheduling, although there may be features in the CPU that support them.
"The operating system kernel (a set of instructions, aka software) executes on the CPU (hardware). A scheduling algorithm in the kernel of the operating system chooses which thread to execute next, and directs the CPU to begin executing the next instruction in that chosen thread".
Clarification: my understanding of "a schedulable unit of CPU time slice" is "a unit that can be scheduled during a given CPU time slice" (since if "schedulable unit" would be a time then the question does not make much sense to me).
Based on this, put it shortly, "a schedulable unit of CPU time slice" for a given logical core can be seen as a software thread (more specifically its execution context composed of registers and process information).
Operating systems scheduler operates on tasks. Tasks can be threads, processes, or other unusual structure (eg. dataflows).
Modern mainstream operating system mainly schedule threads on processing units (typically hardware threads also called logical cores). You can get more information about how the Windows scheduler works in the Microsoft documentation. The documentation explicitly states:
A thread is the entity within a process that can be scheduled for execution
On Linux, the default scheduler, CFS, operates on task (ie. task_struct data structure). Tasks can be a thread, a group of threads or a process. This was done that way so to make the scheduler more generic and also because this scheduler was designed long ago, when processors had only 1 core and people focused on processes rather than thread. The multi-core era since caused applications to use a lot of threads so to use available cores. As a result, nowadays, it is generally threads that are actually scheduled AFAIK. This is explained in the famous research paper The Linux Scheduler: a Decade of Wasted Cores (which also explain a bit how the CFS operate regarding the target processor).
Note that the term "process" can sometime refer to a thread since threads are sometime called "lightweight processes" and basic processes are sometime called "heavy processes". Processes can even be a generic term for both heavy and lightweight processes (ie. threads and actual processes). This is a very confusing terminology and a misuse of language (like the term "processors" sometimes used for cores). In practice, this is often not a problem in a specific context since threads and processes may be used interchangeably though (in such a case, people should use a generic term like "tasks").
As for "a schedulable unit of CPU time slice" this is a bit more complex. A simple and naive answer is: a thread (it is definitively not processes alone). That being said, a thread is a software-defined concept (like processes). It is basically a stack, few registers, and a parent process (with possibly some meta-information and a TLS space). CPUs does not operate directly on such data structure. CPU does not have a concept of thread stack for example (it is just a section of the virtual process memory like any other). They just need an execution context which is composed of registers and a process configuration (in protected mode). For sake of simplicity, we can say that they execute threads. Mainstream modern x86 processors are very complex, and each core is often able to run multiple threads at the same time. This is called simultaneous multithreading (aka. Hyper-Threading for Intel processors). x86 physical cores are typically composed of two logical threads (ie. hardware threads) that can each execute a software threads.