Compiled binaries capable to be loaded in the process address space in runtime.
Dynamic Libraries are compiled binaries capable of being loaded/unloaded into/from the process address space at runtime.
In the Unix world, they are called shared libraies; in the Windows world, they are known as DLLs (dynamically-loadable libraries).
They can be loaded simultaneously into multiple processes, saving RAM and disk space.
Their file format is similar to, or even the same as, that of binary executables.