what is the actual definition of electric charge ?
What is the actual definition of mass? What is the actual definition of temperature?
All three represent properties of materials. The origin of the properties is not known, but it is possible to measure and compare different materials based on those three (and most likely other) properties.
Elementary charge is measured (implicitly through experiments), not defined.
Fwiw, recently another member here, clem, pointed out that in Gaussian units, charge is actually defined in terms of mass, length and time, and has units of mass(in grams)^1/2 x length (in cm)^3/2 x time (in seconds)^-1.