A system without (full-fledged) OS, like an MCU. When running on a baremetal system, MicroPython effectively becomes its user-facing OS with a command interpreter (REPL).
A PCB board. Oftentimes, the term is used to denote a particular model of an MCU system. Sometimes, it is used to actually refer to MicroPython port to a particular board (and then may also refer to “boardless” ports like Unix port).
CPython is the reference implementation of Python programming language, and the most well-known one, which most of the people run. It is however one of many implementations (among which Jython, IronPython, PyPy, and many more, including MicroPython). As there is no formal specification of the Python language, only CPython documentation, it is not always easy to draw a line between Python the language and CPython its particular implementation. This however leaves more freedom for other implementations. For example, MicroPython does a lot of things differently than CPython, while still aspiring to be a Python language implementation.
General-purpose input/output. The simplest means to control electrical signals. With GPIO, user can configure hardware signal pin to be either input or output, and set or get its digital signal value (logical “0” or “1”). MicroPython abstracts GPIO access using machine.Pin and machine.Signal classes.
GPIO port
A group of GPIO pins, usually based on hardware properties of these pins (e.g. controllable by the same register).
Microcontroller. Microcontrollers usually have much less resources than a full-fledged computing system, but smaller, cheaper and require much less power. MicroPython is designed to be small and optimized enough to run on an average modern microcontroller.

MicroPython is (usually) distributed as a single executable/binary file with just few builtin modules. There is no extensive standard library comparable with CPython. Instead, there is a related, but separate project micropython-lib which provides implementations for many modules from CPython’s standard library. However, large subset of these modules require POSIX-like environment (Linux, MacOS, Windows may be partially supported), and thus would work or make sense only with MicroPython Unix port. Some subset of modules is however usable for baremetal ports too.

Unlike monolithic CPython stdlib, micropython-lib modules are intended to be installed individually - either using manual copying or using upip.

MicroPython port
MicroPython supports different boards, RTOSes, and OSes, and can be relatively easily adapted to new systems. MicroPython with support for a particular system is called a “port” to that system. Different ports may have widely different functionality. This documentation is intended to be a reference of the generic APIs available across different ports (“MicroPython core”). Note that some ports may still omit some APIs described here (e.g. due to resource constraints). Any such differences, and port-specific extensions beyond MicroPython core functionality, would be described in the separate port-specific documentation.
MicroPython Unix port
Unix port is one of the major MicroPython ports. It is intended to run on POSIX-compatible operating systems, like Linux, MacOS, FreeBSD, Solaris, etc. It also serves as the basis of Windows port. The importance of Unix port lies in the fact that while there are many different boards, so two random users unlikely have the same board, almost all modern OSes have some level of POSIX compatibility, so Unix port serves as a kind of “common ground” to which any user can have access. So, Unix port is used for initial prototyping, different kinds of testing, development of machine-independent features, etc. All users of MicroPython, even those which are interested only in running MicroPython on MCU systems, are recommended to be familiar with Unix (or Windows) port, as it is important productivity helper and a part of normal MicroPython workflow.
Either MicroPython port or GPIO port. If not clear from context, it’s recommended to use full specification like one of the above.
(Literally, “micro pip”). A package manage for MicroPython, inspired by CPython‘s pip, but much smaller and with reduced functionality. upip runs both on Unix port and on baremetal ports (those which offer filesystem and networking support).