A track day is an organised event in which non-members are allowed to drive or ride around established motor racing circuits, or alternatively (though far less common) on closed or disused airfields.
Criteria for being eligible to participate is usually the holding of a driving licence for the vehicle in question or the appropriate racing license for the event can also be used and the payment of a fee.
There are varying formats for the proceedings, but they usually consist of two or three groups loosely corresponding to an individual's level of experience and/or how quick they are, (e.g. novice/intermediate/advanced or beginner/experienced).
Whatever the interpretation, primarily track days are all about having fun, whether motorbike or car, the emphasis is on enjoyment in a controlled and suitable environment.
This is usually controlled by an organiser by populating the event with fewer participants, albeit usually at a higher price, with instructor guidance facilities usually available.