drop-in

/D R AA1 P IH0 N/
noun
  1. 1

    One who casually drops in.

  2. 2

    An informal event that does not require booking in advance.

adjective
  1. 1

    Provided for short-term use

  2. 2

    Fit to substitute some element in a complex system without changes to the existing infrastructure

Source: https://en.wiktionary.org/wiki/drop-in