Definition:Electric Charge/Polarity/Convention

Definition
It appears at first glance that the definition of the sign of an electric charge is both arbitrary and circular.

That is:
 * a positive electric charge is the type of electric charge that can be found on a proton

while:
 * a proton is defined as carrying a positive electric charge.

Early in the development of the theory of electrostatics, it was noticed that there were two types of electric charge.

What they were called was indeed arbitrary at that point in time; one type was called positive, and the other type was called negative.

When the proton and electron were discovered, the signs of their electric charges were identified, and subsequently used to define the sign of an arbitrary electric charge.

Hence the fact that a proton is positive and an electron is negative is simply the result of a convention.