The electronic structure and bonding in the ${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$ crystal is completely elucidated by first-principles self-consistent band-structure and optical calculations. Results agree well with experiments. The unique type of bonding in ${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$ is emphasized; it does not support the fully ionic picture and is best descibed as ${\mathrm{Y}}_{2}^{+2.16}$${\mathrm{O}}_{3}^{\mathrm{\ensuremath{-}}1.44}$.