Imposte (impost)

The start or birth of an arc (arch).
In this case a trapezoidal ridge which shows where the (mostly rectangular or square) pilier (pillar) or colonne (column) ends and where the arch supported from the pillar starts.
The picture shows clearly two imposts in the small transept of Cluny III.
See also dosseret.

Abbatiale - Cluny

Infirmerie (infirmary)

The infirmary in a monastère (monastery).
The picture shows a ward in the hospital that belonged to the monastery of Canterbury.

Hospital of Saint-Thomas - Canterbury

Intrados (intrados)

The concave inside of an arc (arch) or voussoir, this contrary to the extrados, the convex top or outside of an arch or voussoir.

 Terminology :
1. Clé d'arc ou clef d'arc (keystone)
2. Claveau, ou voussoir (voussoir)
3. Extrados (extrados)
4. Imposte, en haut du piédroit (impost)
5. Intrados ou douelle
6. Flèche (hauteur intérieure de la partie courbe de l'arc) (rise)
7. Portée (distance entre les deux appuis verticaux ou piédroits) (clear span)
8. Écoinçon (spandrel)

Intrados - Wikipedia

Intrados - Wikipedia