Clipboard manager
The program that keeps what you copied alive after the source program closes, and usually holds a history of recent clips — a necessity under X11, where the clipboard is not a buffer but a promise.
In X11 nothing is stored when you copy. The program merely announces that it owns the CLIPBOARD selection, and whoever pastes asks the owner for the content at that moment. Close the program and the owner disappears, taking the text with it. A clipboard manager fixes this by being a client that takes over: the freedesktop.org specification has it own the CLIPBOARD_MANAGER selection, and a program about to exit requests the SAVE_TARGETS target, whereupon the manager fetches the content in every format and steps in as the new owner. xclipboard from X11's own toolbox did roughly this long before the specification existed; KDE's Klipper, Xfce's Clipman, Parcellite, CopyQ and GPaste added what users actually wanted, a list of recent clips to pick from.
Other systems got there late or not at all. Windows cut and pasted with a single buffer for thirty years before Win+V brought a history in October 2018, and macOS still has one clipboard unless you install something. Under Wayland the question came back, because the compositor, for security reasons, does not let other clients read selections: the data-control protocol, first as a wlr extension and later as ext-data-control, exists precisely so tools like cliphist can keep doing what X11 always permitted.