stay behind

Short definition from Wiktionary

Noun

1. 1
   Alternative form of stay-behind.
2. 2
   One who remains when expected to leave or after others have left.
3. 3
   An agent who lives in a foreign country, and can be activated in certain circumstances.

Verb

1. 1
   Used other than figuratively or idiomatically: see stay, behind.
2. 2
   To remain where one is, whilst others leave.
3. 3
   To remain in a classroom or school at the end of teaching, especially to receive punishment.