Four.

A surname.

plural of Quadri

A polygon with four sides. / An area defended by four fortresses supporting each other.

Having four sides.

(US, modern British and Australian, short scale) A thousand trillion (logic: 1,000 × 1,000^4): 1 followed by fifteen zeros, 10¹⁵.

(figuratively, slang, hyperbolic) Any very large number, exceeding normal description.

(geometry) A quadrilateral whose vertices all intersect a single circle.

(rare) A grouping of four things. / (US) A government committee of four economic experts.

(anatomy) A muscle having four heads, especially the large extensor at the front of the thigh.