The physical distance in space covered by one complete cycle of a periodic waveform as it passes through some medium such as air. 
Wavelength for sound propagating in air can be easily calculated using the following equation: 
Wavelength = Speed of Sound/Frequency 

For example, if we want to calculate the wavelength of one cycle of a 30 Hz tone we would evaluate the following equation: 

344 metres / 30 Hz = 11.46 metres 
1130 feet / 30 Hz = 37.66 feet