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