Source : Free On-Line Dictionary of Computing

contention slot
     
        (Or contention period).  Minimum time a {host} must transmit
        for before it can be sure that no other host's {packet} has
        collided with its transmission.  If the maximum propagation
        delay from one host to any other is T, then a host that starts
        to transmit at time t0 may collide with a host that starts
        just before t0 + T.  The first host will not detect the
        collision until time t0 + 2T.