drop-in

Short definition from Wiktionary

Adjective

1. 1
   Provided for short-term use.
2. 2
   Fit to substitute for some element in a complex system without changes to the existing infrastructure.
3. 3
   Allowing people to drop in; that is, to visit casually, without an appointment.
4. 4
   Of the pitch: prepared away from the ground or venue, and dropped into place only when a match is to be played.

Noun

1. 1
   One who casually drops in (visits unannounced or without appointment).
2. 2
   An informal event that does not require booking in advance.
3. 3
   A place that can be visited casually, without an appointment.
4. 4
   The act of dropping in; that is, taking a wave that another surfer is already riding.
5. 5
   A pitch that is prepared away from the ground or venue, and dropped into place only when a match is to be played.
6. 6
   A message or advertisement inserted into other material.

Verb

1. 1
   Used other than figuratively or idiomatically: see drop, in.
2. 2
   To arrive casually and unannounced, with little or no warning; also, to visit without an appointment.
3. 3
   To paddle into and take off on a wave another surfer is already riding.
4. 4
   To fall in; to cave in.
5. 5
   To install components, particularly car engines.