What does the 'V' stand for in ECG electrode names?
Short answer
V stands for voltage.

Background
The various leads in an ECG montage are shown below:


ECG electrodes. Source: American Heart Association 

I am not an expert in ECGs, but as far as I can see, RA, LA and LL are return electrodes as they are situated far away from the heart. In other words, they act as reference electrodes. RL is the ground, basically to correct for baseline shifts. This leaves the V electrodes, which are the chest leads, or precordial leads, and are situated right above where the action is, i.e., the heart. The ECG is determined between the V electrodes and a distant reference, with a ground added. The V electrodes are the active electrodes, picking up the signal. The others are references and a ground only.