What characteristic defines a sine wave?

A sine wave is defined by its smooth, regular oscillation, making it the fundamental waveform used in alternating current (AC) signal representation. This characteristic is crucial in various applications, including signal processing, communication technologies, and audio engineering. The sine wave has a clear periodic nature, allowing it to repeat continuously over time with a constant frequency, amplitude, and phase. The mathematical representation of a sine wave confirms its smooth transitions between its maximum and minimum values, emphasizing its importance in illustrating phenomena that vary harmonically, such as sound waves and electrical power generation.

The other options do not accurately capture the defining traits of a sine wave. Continuous increase in amplitude refers to a waveform like a ramp or sawtooth wave, while irregular and non-repeating signals might describe complex waveforms or noise. A square waveform has abrupt changes and retains constant amplitude for short periods, which contrasts sharply with the smooth transitions of a sine wave. Therefore, the correct characterization of a sine wave highlights its periodic, smooth, and repetitive nature, aligning with the characteristics of AC signals.