VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.
Well unless someone can confirm the current rating for the D1 series polarity protection diode use with external power, I would not count on being able to safely pass 2 amps of current out the Vin pin. On my Arduino clone board the D1 diode is a 1N4004 type which carries a 1 amp continous rating.
I'm working on a very small 7 DOF robotic arm made from some 5g servos.