The Free On-line Dictionary of Computing (30 December 2018):

contention slot

    In a communication system where only one node at a
   time may transmit successfully on a shared channel, the contention
   slot or contention period is the time a node must wait before it
   can be sure that no other node's transmission has collided
   (collision) with its transmission.

   If node A starts to transmit at time t0 and then another node
   starts to transmit just before it recieves A's transmission at
   time t0 + T, then the transmissions will collide but node A will
   not detect the collision until time t0 + 2T.  The contention slot,
   2T, for nodes seperated by the maximum propagation delay thus
   determines how much data the node must be prepared to re-transmit
   in the event of a collision.

   (2014-11-06)