VDC is an abbreviation for “Volt DC”. DC stands for “direct current”, which means that the voltage is constant (in contrast to alternating current, alternating current, in which the voltage oscillates constantly between positive and negative polarity).
So “VDC power” means nothing in itself, but “12 VDC power” means power that is supplied with 12 volts DC. (According to some style guides, it is more correct to write “V DC” instead of “VDC”, but “VDC” is common in industry.) If you want to talk about DC but don’t want to supply voltage, just say “DC”. And a note about the term “DC”: don’t think too much about what the words actually mean. DC really isn’t “more direct” than AC, and both terms usually refer to voltage, not current. But we stay with them.
