MSVCRT, the Microsoft Visual C/C++ Runtime library, is also 'the runtime library'. It was the runtime library for Microsoft's C/C++ compiler. In the days when there were multiple C/C++ implementations for Win32 (which still exist, if one is willing to dig up Watcom C/C++ or some such) there would be different runtime library DLLs for the different C/C++ vendors, even for different versions of their products.

Runtime libraries for C/C++ provide two general sets of stuff: the stuff mandated for the Standard C and Standard C++ libraries, and the stuff that is needed by the basic mechanics of the language.

The former is everything from abort() to wscanf(). The latter is a bunch of internal functions, calls to which the compiler inserts in order to do stuff. This is basically the split nowadays between UCRT and VCRUNTIME.

In the days of programming targetting the 80486SX without an 80487 present, for instance, every piece of floating point arithmetic was not a machine instruction but a call to a runtime library routine that did the floating point operation longhand using non-FPU instructions. Other runtime functionality over the years has included doing 32-bit or 64-bit integer arithmetic on 16-bit and 32-bit architectures where this was not a native word size, functions to do stack checking in the function perilogue, and functions to do C++ run-type type checking and exception processing.

This pattern is followed by other (compiled) programming languages. Naturally, the programming languages do not necessarily have any relation to the Standard C or Standard C++ libraries, nor do they generate code that needs the same helper functions for stuff as C/C++ code does. (But the situation is complicated by the POSIX API and the old C language bindings for the MS-DOS system call API, some of which another programming language might also allow program code to use.)