It corresponds to speed at rated torque, rated current, and rated voltage conditions at the temperature rise specified in the rating. It is the maximum speed at which motor can operate under constant torque characteristics or the minimum speed to operate at rated power.
"When at full armature volts and current, it is possible to reduce the field current and the motor speed will increase. The torque output from the motor will reduce in proportion with the reduction of field current. The area of operation is sometimes called the constant power area of operation. The area below base speed is called the constant torque area of operation. "