Sentence:  Results of this experiment include the frequencies of the two strong spectral <WSD>lines</WSD> by which OH may be identified in interstellar gas ; the frequencies are 1665.32 and 1667.36 * * f , with an uncertainty of 0.10 * * f . 
Meaning of the word marked with <WSD> tags:  a single frequency (or very narrow band) of radiation in a spectrum