As a paramedic who does EKGs every day, I can tell you it stands for "VECTOR" meaning the direction the electrode monitors electricity from the heart.
RA, LA, RL, and LL electrodes are ALL the principle electrodes for monitoring cardiac activity. These are referred to as the "limb leads" and are bipolar leads. The direction these leads monitor is determined by the polarity of the lead, which CAN change. For example, with Lead I the right shoulder is negative and the left shoulder is positive; but in Lead III the left shoulder is negative while the left leg is positive. Electricity flows from negative to positive. When it flows towards the positive electrode, the deflection on the EKG is upward. When it flows away from the negative electrode the deflection on the EKG is downward.
The augmented vector leads are so-called because they have one positive electrode and the negative electrode is augmented. These are aVR, aVL and aVF where R, L and F stand for Right, Left, and Foot respectively. The V stands for vector here. In aVR, the left leg lead and left shoulder lead BOTH provide the negative pole for the EKG while the right shoulder is positive. The augmented vector leads require THREE poles to work: two negative and one positive.
The precordial leads, asked about in the original question, are UNIPOLAR leads. The computer inside the EKG uses the position of all the unipolar leads to create a "dummy" negative point at or near the center of the heart. Each Lead then becomes its own positive, and assumes that it is looking to a negative pole at the center of the heart. These are numbered V1 through V6, and the V stands for Vector. Vector 1, Vector 2, Vector 3 and so on.
One could do an 32-Lead EKG with the 3 Limb Leads, 6 Left Anterior Precordial Leads, 6 Left Posterior Leads, 6 Right Posterior Leads and 6 Left Precordial Leads. It is possible to also do a 64-Lead EKG. But the most common arrangements are 3, 4, 12, and 15 Lead EKGs.