The fourth letter of the alphabet: called ثَآءٌ and ثَا [respecting which latter see the letter ب]: the pl. [of the former] is ثَآءَاتٌ; and [of the latter,] أَثْوَآءٌ and أَثْيَآءٌ. (TA in باب الالف اللّينة.) It is one of the letters termed مَهْمُوسَة [or non-vocal, i. e. pronounced with the breath only, without the voice], and of those termed لِثَوِيَّة [or gingival], which are ث and ذ and ظ. (TA at the commencement of ف.)

It is sometimes substituted for ف as in the instance of حُثَالَةٌ and حُفَالَةٌ; and for س as in the instance of جُثْمَانٌ and جُسْمَانٌ; and for other letters. (TA in the latter place.)

[As a numeral, it denotes Five hundred.]