In Unix computing, Ion is a tiling and tabbing window manager for the X Window System. It is designed such that it is possible to manage windows using only a keyboard, without needing a mouse. It is the successor of PWM and is written by the same author, Tuomo Valkonen. Since the first release of Ion in the summer 2000, similar alternative window management ideas have begun to show in other new window managers: Larswm, ratpoison, StumpWM, wmii, xmonad and dwm.

First versions of Ion were released under the Artistic License, Ion2 and the development versions of Ion3 were released under the GNU Lesser General Public License (LGPL). However, the first release candidate of Ion3 included a license change to a custom license based on the LGPL (specifically modified versions must not use the name ion).

Since version 2, Ion has been scriptable in Lua.

As of September 17, 2009, Valkonen states he is unlikely to continue development of Ion by himself.

The official home page went offline in early 2010.^{[citation needed]}

A fork, Notion (Not-ion), is being maintained.

The Notion (Not-ion) fork is actively maintained with packages available for the Linux distributions gentoo, Debian, Arch, SUSE and Fedora as well as NetBSD and Solaris (Solaris 10, OpenSolaris and OpenIndiana).

Window managers similar to ion include awesome, dwm, i3, larswm, and xmonad.