Sentence:  The study of the radio emission of the moon and planets began with the detection of the thermal radiation of the moon at 1.25 - cm <WSD>wave</WSD> length by Dicke and Beringer ( 1946 ) . 
Meaning of the word marked with <WSD> tags:  the distance (measured in the direction of propagation) between two points in the same phase in consecutive cycles of a wave