One who casually drops in (visits unannounced or without appointment).
An informal event that does not require booking in advance.
A place that can be visited casually, without an appointment.
(surfing) The act of dropping in; that is, taking a wave that another surfer is already riding.
Provided for short-term use.
(manufacturing) Fit to substitute for some element in a complex system without changes to the existing infrastructure.
(of a place or facility) Allowing people to drop in; that is, to visit casually, without an appointment.