Definition of "drop-in"

drop-in

adjective
not comparable
1. Provided for short-term use.
2. (manufacturing) Fit to substitute for some element in a complex system without changes to the existing infrastructure.
  Quotations
  An alternative to cellulosic based second- generation biofuels are drop-in biofuels, that is, fuels such as synthetic gasoline that can be put into fuel tanks and pipelines with no modifications.
  2012, Robert Mendelsohn, Ariel Dinar, Handbook on Climate Change and Agriculture, page 276
3. (of a place or facility) Allowing people to drop in; that is, to visit casually, without an appointment.
noun
plural drop-ins
1. One who casually drops in (visits unannounced or without appointment).
2. An informal event that does not require booking in advance.
3. A place that can be visited casually, without an appointment.
4. (surfing) The act of dropping in; that is, taking a wave that another surfer is already riding.
  Quotations
  Drop-ins can and often do happen accidentally. Sometimes you're so focussed on your take-off that you just don't see the other rider.
  2004, Bill Mattos, Kayak Surfing, page 37