H-alpha (Hα) emission is the red visible spectral line created by a hydrogen atom when an electron falls from the third lowest to second lowest energy level.  This transition corresponds to a wavelength of 656.28 nm (red light) and is the first transition in the Balmer series.  The spectrum below shows the four visible hydrogen emission spectrum lines that make up the Balmer series, with the H-alpha transition in red on the far right.