'single-frequency' originates from the combination of 'single,' meaning 'one,' and 'frequency,' which comes from Latin 'frequentia,' meaning 'a crowd or multitude.'

'Frequency' evolved from the Latin 'frequentia' to the modern English 'frequency,' while 'single' has remained largely unchanged from Old English 'sengel.'

Initially, 'frequency' referred to 'a crowd or multitude,' but over time it evolved to mean 'the rate at which something occurs over a particular period.'