ʿilm al-Ḥurouf or science of letters is an Arabic esoterism science which is interested in the symbolism that the letters of the Arabic alphabet may contain. It is often considered an occultist science.
Etymology
ʿilm al-Ḥurouf (arabe:علم الحروف) is composed with two words ʿilm which means "science", and al-Ḥurouf, plural form of the word Ḥarf which may be translated into "letter".
References
ʿilm al-Ḥurouf Wikipedia(Text) CC BY-SA