Trivector meter is used to measure three types of 
power.that is active power,reactive power and apparent 
power.

It is used to measure voltage, current & power of a circuit.
As it measures three components so it is called trivector 
meter

It is a device that measures the amount of electrical 
energy supplied to or produced by residence, business, or 
machine.
The most common type is more properly known as a  kilowatt 
hour meter or a meter. When used in electricity retaining, 
the utilities record the values measured by these meters to 
generate an invoice  for the electricity. They may also 
record other variables including the time when the 
electricity was used.
It is called as tri-vector meter because it measures energy 
consumption of the three phase lines R, Y, B which are 120 
phase difference from each other.It measures the 
consumption in terms of the active enrgy, reactive energy, 
apparent energy, power factor.