We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass selected terbium dimers in argon matrices. An absorption band found between 620-680nm was assigned to terbium dimers. The Raman excitation profile of dimer frequencies resembles closely this absorption spectrum. Unlike other lanthanide dimers, resonance Raman spectra of terbium dimer, obtained by excitation into this region with tunable dye laser radiation, gives two progressions with one up to 7 Stokes transitions, and the other up to 4 transitions. We interpret these to represent two distinct electronic states X and A. For the ground (X) state, we obtain w_e = 137.6 ± 0.4 cm^-1 with w_ex_e = 0.31 ± 0.05 cm^-1, leading to a spectroscopic dissociation energy of 1.9 ± 0.3 eV, force constant k_e = 0.88 ± 0.01 mdyne/Å. The first electronic (A) state has a origin at 313.9cm^-1 with a progression in w_e = 136.3 ± 0.2 cm^-1 and w_ex_e = 0.14 ± 0.05 cm^-1, leading to an excited state spectroscopic dissociation energy of 4.1 ± 1.2eV.