Shiva Purana
वृत्तं वा चतुरस्रं वा बिन्दुमर्धेन्दुमेव वा ॥ ललाटे यादृशं पुण्ड्रं लिखितं भस्मना पुनः ॥ ३३ ॥
It may be a circle or a square or a dot or a half moon. The kind of puddle on his forehead that was written again with ashes.
english translation
vRttaM vA caturasraM vA bindumardhendumeva vA ॥ lalATe yAdRzaM puNDraM likhitaM bhasmanA punaH ॥ 33 ॥
hk transliteration by Sanscript