A quick-release air chuck with a finger pull tab.

The chuck is the end that clips onto the valve stem. Quick-release means it seats and releases in a second rather than threading on, and the pull tab is there so you can release it wearing gloves, or with cold hands, without fighting it.

Buy one as a replacement for a chuck that has been crushed or started leaking, or as a spare — it is the smallest part of an air system and the one most likely to go missing.