We propose that the carbon dimer defect CBCN in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 3.8 eV is close to the experimental value, and the deduced Huang-Rhys factor of S ≈ 2.0, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized π-type defects states, with a very short radiative lifetime of 1.7 nanoseconds, in very good accord with experiments.

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