A pure tone is a tone with a sinusoidal waveform, i.e. a sine or cosine wave. This means that regardless of other characteristic properties such as amplitude or phase, the wave consists of a single frequency. Sine and cosine waves are the most basic building blocks of more complex waves, and as additional frequencies (i.e. additional sine and cosine waves having different frequencies) are combined, the waveform transforms from a sinusoidal into a more complex shape.
A sine wave is characterised by its frequency, the number of cycles per second—or its wavelength, the distance the waveform travels through its medium within a period—and the amplitude, the size of each cycle. A pure tone has the unique property that its wave-shape and sound are changed only in amplitude and phase by linear acoustic systems.
Disclaimer: Sine wave sounds should be used responsibly such as: in a testing environment for calibration purposes. They have been known to cause noise-induced hearing loss at lower volumes than other noises so use at your own risk.