Just check here: Why V=IR is not Ohm’s Law, and why that matters
What is V=IR?
This formula is the definition of electrical resistance (often stated as R=VI, but it’s the same thing, just rearranged). The SI units of the quantities voltage, current and resistance are volts (V), amperes (A) – amps for short, and ohms (Ω), respectively. So if a voltage of 10 V causes a current of 2 A in a resistor, then its resistance is 5 Ω.
Why is it not the Ohm's Law?
Resistance doesn't have to be always constant as in an old-style filament light bulb, it can vary due to factors such a temperature. A light bulb just has a resistance that changes with current, unlike the constant resistance of a resistor. So in each instant of time V=IR, but Ohm's Law does not hold due to R being variable.