Num lock (also called Numpad lock or Numerics lock) is a lock key introduced by IBM to permit the numeric keypad to also provide navigation functionality. With Num lock engaged, the numeric keypad would enter numbers. With Num lock disengaged, it would provide cursor keys and other navigation functionality.
With the introduction of the IBM Enhanced Keyboard, with its dedicated navigation clusters, Num lock no longer served any purpose on full-size desktop keyboards except backwards compatibility. However, the functionality has been retained ever since. Num lock is typically left permanently enabled on desktop computers running Microsoft Windows and Linux; often with Windows, it must be enabled when a Windows user account is created for the first time, and will remain enabled from that point on.
Apple have never provided numeric keypad-based cursor navigation on Macintosh computers, and the num lock legend on some Macintosh keyboards is simply there for compatibility with PC applications (for example PC emulation); under all variants of Mac OS, the key serves as the clear key.
Many compact keyboards, for instance on laptops, lack the space for a dedicated numeric keypad but many people are accustomed to entering numbers via the numeric keypad instead of the number row. Many such compact keyboards therefore offer a special keyboard layer for the keypad on the regular alphanumeric keyboard and that layer is often enabled by a key that is called Num Lock. Some tenkeyless keyboards also provide this functionality, including the Topre Realforce.
Some laptops lack a num lock LED, and it is not possible to visually determine whether num lock is engaged (changes in state are indicated momentarily by on-screen notification). This can cause problems for password entry at the logon desktop, when the keyboard is typing numbers instead of letters, and the user cannot tell.
Over PS/2 and USB, the state of the locking keys is host-controlled and the keyboard needs to be smart enough to send virtual Num lock key presses to influence the host's state to match the keyboard's intended state - but this does not always work. To avoid that complexity and the risk for error, some PC keyboards therefore instead have a separate Pad lock key only to change the layer, with a separate Num lock key that works as on a regular full-size keyboard switching between numeric entry and navigation.
Almost all Num lock key have textual legends. ISO/IEC 9995-7 mandates the symbol ⇭ but it is very unusual.
Lock lights tend to have a text legend, the word "Num", the word "Num" with a downwards arrow, the digit 1, the digit 1 with a downwards arrow or the digit 1 scribed inside a padlock symbol.