Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. In other words, it is a type of electric current characterized by the fact of inverting the electric polarity continuously over time. Basically, unlike the direct current, in which the polarity is fixed and invariable, in alternating current the positive pole becomes negative and vice versa with an alternation (hence the name) which occurs with a fixed frequency (typically 50 Hz or 60 Hz).
The polarity inversion, however, does not occur abruptly but with a progressive variation in a pattern called sinusoidal, in which the value of current (and therefore voltage) starts at zero and increases gradually in a given direction, reaches its maximum value and then decrease to zero and return in the opposite direction with the same trend (the cycle repeats).