drop-in

Adjective

• Provided for short-term use.
• Fit to substitute for some element in a complex system without changes to the existing infrastructure.
• Allowing people to drop in; that is, to visit casually, without an appointment.

Noun

• 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.
• The act of dropping in; that is, taking a wave that another surfer is already riding.