Wiktionary
n. (context arts chiefly calligraphy English) An Arabic script style.
Wikipedia
Thuluth ( sols, Turkish: Sülüs, from "one-third") is a script variety of Islamic calligraphy invented by Ibn Muqlah Shirazi. The straight angular forms of Kufic were replaced in the new script by curved and oblique lines. In Thuluth, one-third of each letter slopes, from which the name (meaning "a third" in Arabic) comes. An alternative theory to the meaning is that the smallest width of the letter is one third of the widest part. It is an elegant, cursive script, used in medieval times on mosque decorations. Various calligraphic styles evolved from Thuluth through slight changes of form.
Usage examples of "thuluth".
Their names are just as celebrated for Thuluth and Talik writing as were formerly those of Ibn Bawwab, of Ibn Hilal, and of Yakut are for Naskhi.