1) Curve 2) Curved shape 3) Inner curve of a vault 4) The interior curve of an arch 5) Tighter of two curves 1) Interiority 
undersurface of an arch or vault
• (n.) The interior curve of an arch; esp., the inner or lower curved face of the whole body of voussoirs taken together. See Extrados.
The inside curved soffit of an arch. The opposite face is known as the extrados. 
In an arch or vault is the inner surface of the arch barrel, i.e. the inner (concave) curve of the barrel.
inner face of voussoirs. See arch. (Illustration) 
The inner curve or underside of an arch. Also known as a soffit.
A construction line denoting inside limit of the arch. It is used in the setting out. Not be be confused with the soffit which is a surface.; 
In·tra'dos noun [ French, from Latin intra within + French dos the back, Latin dorsum . Confer Extrados .] (Architecture) The interior curve of an arch; esp., the inner or lower curved face of the whole body of voussoirs taken together. See Extrados . 
In architecture an intrados is the interior curve of an arch. The term is especially applied to the inner or lower curved face of the whole body of voussoirs taken together.
the lower or inner curve of an arch. Often contrasted with extrados
The underside of an arch or a vault. Also see: extrados.
[n] - the interior curve of an arch
The underside, or soffit, of an arch.
the interior curve or surface of an arch or vault. Cf. extrados.