Electric charge is a property of matter that determines how objects participate in electric interactions. Charge comes in two signs, conventionally called positive and negative.

Charges with the same sign repel, while charges with opposite signs attract.

The sign of a charge identifies which kind of electric interaction it participates in; it does not indicate how much charge is present. Positive and negative charges can each have different magnitudes.

The SI unit of charge is the coulomb ($\mathrm C$).

Conservation of charge

Electric charge can move from one object to another, but the total charge of an isolated system is conserved. If one part of a system gains negative charge, another part must lose the corresponding negative charge or gain an equivalent positive charge.

Mobile and localized charge

In a conductor, some electric charge can move through the material relatively freely. In an insulator, charge is much more strongly localized.

Electric charge is therefore both a property carried by matter and a conserved quantity that can be redistributed through interactions. More microscopic units describe the elementary charge carried by particles and the discrete charge changes of atoms and ions.