What is the name of the equation P=IV?
Informally, this is referred to as the "power law" or "power formula". It seems that there is a widespread practice to refer to it, incorrectly, as "Watt's law". 


  However, note that the first term potentially conflicts with a widely used meaning of "power law" which describes any situation in which two quantities are related via any any power higher than 1: quadratically, or cubically, and so on. For example, if a doubling in some independent variable X causes some dependent variable Y to quadruple, then X and Y are linked by a power law.


It closely corresponds to what Wikipedia calls Joule's first law:

$$Q = I^2 \cdot R  \cdot t$$

Watts is just \$P = \displaystyle\frac{Q}{t}\$, which reduces it to \$P = I^2R\$, and by Ohms we get all the variants.

It looks like there is justification in just calling all variations of the electric power law "Joule's First Law".

However, people (who otherwise know a thing or two about electronics, as well as Ohm's Law) might not understand "Joule's First Law". In casual conversation, in which I don't want to get uppity and educate people about terminology, I would call it the "voltage-current product law".