mezzanine
US /ˈmet.sə.niːn/
UK /ˈmet.sə.niːn/

1.
mezzanine, entresol
a low story between two main stories of a building, typically an intermediate floor between the ground floor and the first floor.
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The library has a beautiful mezzanine level with extra seating.
La bibliothèque dispose d'une belle mezzanine avec des sièges supplémentaires.
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We watched the concert from the mezzanine seats.
Nous avons regardé le concert depuis les sièges de la mezzanine.