The onauxclick property is an event handler called when an auxclick event is sent, indicating that a non-primary button has been pressed on an input device (e.g. a middle mouse button).
This property is implemented as part of a plan to improve compatibility between browsers with regards to button behaviours — event behaviour is being updated so that click only fires for primary button clicks (e.g. left mouse button). Developers can then use auxclick to provide explicit behaviour for non-primary button clicks.
Previous to this, click generally fired for clicks on all input device buttons, and browser behaviour was somewhat inconsistent.