intervene

Concise Oxford English Dictionary © 2008 Oxford University Press:
intervene/ɪntəˈviːn/

▶verb

• 1 come between so as to prevent or alter something.

  ■ occur as a delay or obstacle to something being done.

  ■ interrupt verbally.

  ■ Law interpose in a lawsuit as a third party.

• 2 (usu. as adj. intervening) occur in time between events.

  ■ be situated between things.

– derivatives
intervener (also intervenor) noun,
intervenient adjective.

– origin C16 (earlier (ME) as intervention): from L. intervenire, from inter- ‘between’ + venire ‘come’.