"V1" speed is the maximum speed at which the takeoff can be safely aborted; after V1 speed is exceeded, the plane must take off to avoid overrunning the runway. The captain of Flight 159 failed to announce that Flight 159 had achieved V1 speed, and the first officer believed the plane was at or near V1 (rather than having substantially exceeded V1) when he aborted the takeoff.
The maximum speed for a safe abort of a takeoff is "V1", after which the plane must take off to avoid exceeding the runway's length. The captain of Flight 159 failed to announce that Flight 159 had reached "V1" speed, and the first officer believed the plane was at or near "V1" when he aborted the takeoff.