What is an "arc" spectrum ? I sometimes hear about astronomers using an arc spectrum to calibrate observations. For example a "He-Ar arc spectrum". What is an "arc" in this context? I assume it's nothing got to do with angles (arcmin, arcsec, etc.).
An arc spectrum is one produced by a discharge lamp where the discharge is through ionised gas, in the case of He-Ar a mixture of Helium and Argon, which produces a predictable line emission spectrum. 

They are often used to provide a calibration spectrum for spectrometers.