A core dump is a copy of the state of a program or OS. Usually dumped to a file when it crashes.
When a program crashes the contents of its memory may be written to a file. This information can then be used to determine why the crash happened.
This can happen with both regular programs as well as when the OS panic. Such a file contains the core memory and hence it as called a core dump. Other names used for it as system dump and memory dump.
Dump can also be triggered on demand. E.g. with
kill -SIGQUIT process_id.
man 3 abort.