A runtime library is a collection of software programs used at program run time to provide one or more native program functions or services. The runtime library enables a software program to be executed with its complete functionality and scope by providing add-on program resources that are essential for the primary program.
Techopedia explains Runtime Library
The runtime library is primarily the software/programming component of a runtime system. Typically, it consists of many different programs or functions that are commonly used in various programs. These include I/O routines, graphical functions, mathematical functions and more. The runtime library is invoked in all programs. At program runtime, the respective runtime library or function is loaded in the memory until the primary program has finished execution or no longer requires that function.
A dynamic link library is a type of runtime library that dynamically links with programs on its runtime or execution.