That might be "wavelength", written wrong. That refers to the distance between one maximum and the next on a wave - the distance of a full period.
Lengthwave might also be an incorrect term for a longitudinal wave.
"Lengthwave" is not a commonly recognized term or concept. It may refer to a specific concept or theory in a particular field that is not widely known. Additional context would be needed to provide a more accurate explanation.
wave lengthwave lengthWAVE LEnGtH