0_3857 In a narrow-gap semiconductor, or a semimetal with slightly overlapping conduction and valence bands, excitons – electron-hole pairs bound together by Coulomb attraction – spontaneously form. Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of excitons. Excitonic insulators share similarities with the Bardeen-Cooper-Schrieffer superconductor: a distinctive broken symmetry, inherited by the exciton character, and collective modes of purely electronic origin. With only a few candidate materials known, the collective excitonic behavior is challenging to observe in solids, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta₂NiSe₅ disentangling it from the lattice phonons [1-3]. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to noncritical lattice modes is shown to enhance the transition temperature. The work was supported by the NSF-BSF under Grant No. DMR-2105001 and by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program Grant Agreement No. 885413.  [1] Mai Ye et.al., PRB 104, 045102 (2021) [2] P. Volkov et.al., PRB 104, L241103 (2021) [3] P. Volkov et.al., npj Quantum Materials v. 6, 52 (2021)