In fact it is “وحد” which its “و” was changed into “أ” because of alleviation.
Ahad (احد) is sometimes used as a noun (as:احد عشر ) and sometimes is used as adjective. As an adjective it means “uncountable”. Ahad is not synonym of “واحد”. Wherever we say something is “واحد”, it would simply, imply that it does not have the second and third, and so on… . In fact, the word Vahed (واحد) is used for someone or something which is countable. Whereas, Ahad as an adjective, is used for someone or something which is uncountable.
Ahad refers to the Being who is uncountable, unique, and nonpoint. Because God is not in time-space. God is the creator of time and space. Counting only is possible in time-space, where one can be distinguished from another by time or space difference. These concepts are not applicable to God who is not in time-space .
 M. Babaee, Translation and Interpretation of Hamd and Tawheed Surah, pp. 64-67, 2013.