A source which is said to be one color, or monochromatic, will always contain some spread of wavelengths, even if this is relatively small. The linewidth is how big this spread is, and comes from the days of spectrometry when white light was split up into its component colors using line gratings. Often this is defined as the full-width at half maximum, FWHM, Δλ, on a plot of output power against wavelength. Sometimes it is expressed in terms of frequency.
Related Terms: Emission Spectra, Full-Width at Half Maximum (FWHM)