rientrino

Rientrino is an Italian noun used chiefly in architecture, urban planning, and geometry to denote a reentrant angle or indentation—a concave corner formed by the inward recession of a boundary. The term derives from the verb rientrare (to re-enter) with the diminutive suffix -ino, signaling a specific or small form of reentry.

In architecture, rientrini describe internal recesses or jogs in a façade or floor plan, where walls pull back to create a notch. They can influence light, shadow, and circulation, and are sometimes used to create sheltered alcoves or to address structural or aesthetic considerations.

In geometry and technical drawing, a rientrino corresponds to a concave vertex of a polygon; any polygon that includes a rientrino is concave, possessing interior angles greater than 180 degrees at the corresponding vertex. The concept is commonly discussed in Italian literature on polygonal geometry and architectural geometry, where precise terminology helps distinguish concave features from convex ones.

Etymology: from rientrare 'to re-enter' plus the diminutive -ino, indicating a small or particular instance of reentry.

Because rientrino is specialized, it is primarily used by architects, urban planners, and mathematicians working with planar geometry. In English-language texts, the closest equivalents are reentrant angle or concave vertex.