tmux
Terminal multiplexer. Nicholas Marriott, 2007. Successor to GNU Screen (less buggy, more modern config). Sessions survive SSH disconnect, multiple windows + panes per session.
Default prefix Ctrl+b (many remap to Ctrl+a for screen-like behavior). Sessions can be detached + reattached. Used by server admins who SSH in and want persistent long-running terminals. Configuration in ~/.tmux.conf. Modern alternative: Zellij (Rust, more beginner-friendly, layouts via WASM plugins), screen (GNU original, less popular). Often combined with Neovim for a full CLI workflow.