Entresol is a low story between two main stories of a building, generally above the first story, but in London most usually between the ground-floor and the first story. Mezzanine is another name for the entresol, as also for its low, broad windows.
Entresol
Chambers's Encyclopaedia, Volume 4: Dionysius to Friction, p. 388
Source scan(s): p. 0399