caesi

Caesi is a Latin adjective meaning bluish-gray. In classical Latin, the term describes a hue between blue and gray, and in later color terminology the idea persists in descriptions of bluish-gray tones. The masculine plural nominative form in Latin is caesi.

In English, the root appears in several color terms derived from caesius, notably caesious and caesiaceous, used to describe bluish-gray tones in botany, zoology, heraldry, and art description.

The root also provided the basis for the chemical element name cesium, or caesium. Discovered by Bunsen and Kirchhoff in 1860, the element’s spectrum displays prominent blue lines, which inspired the name. The spelling cesium is standard in American English, while caesium remains common in other varieties of English.

Although caesi is not a common standalone noun in modern English, its derivatives survive in technical vocabulary and in historical references to Latin color terminology.