A sine wave. The period is the length of time it takes for one cycle to be completed.
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 above figure is a sine wave. Any ac wave that consists of a single frequency has a perfectly sinusoidal shape. Any perfect si- nusoidal ac source has 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 is an almost perfect ac sine wave with a frequency of 60 Hz.