The sine wave

Sometimes, alternating current has a sine-wave, or sinusoidal, nature. This means that the direction of the current reverses at regular intervals, and that the current-versus time curve is shaped like the trigonometric sine function. The waveform in Fig. 1 is a sine wave.
Any ac wave that consists of a single frequency will have a perfect sine waveshape. And any perfect sine-wave current contains only one component frequency. In practice, a wave might be so close to a sine wave that it looks exactly like the sine function on an oscilloscope, when in reality there are traces of other frequencies present. Imperfections are often too small to see. But pure, single-frequency ac not only looks perfect, but actually is a perfect replication of the trigonometric sine function. The current at the wall outlets in your house has an almost perfect sine waveshape, with a frequency of 60 Hz.

Fig. 1: A sine wave. The period is the length of time for one complete cycle.