Literally: One. Unique. Matchless.
A Name of Allah. Allah. The One Who possesses the perfect attributes qualified with singleness and oneness. The One Who is free of and exalted from all the deficient attributes. The One Who does not have any shariks in His Essence.
The words ahad and wâhid denote the single, sole, one, indivisible Being rising Whose number is impossible when attributed to Allah. The oneness here does not mean the first digit of a number sequence but bears the meaning Allah is not a Being consisted of compounded parts, and He has no similars or matches. While deriving from the same root, there are differences concerning usage between the words ahad and wâhid. The Name of Ahad denotes the oneness, unity, singleness of Allah in His Essence. While the Name of Wâhid negates and rejects the shariks in His attributes encompassing the universe and the creatures within it.
Al-Ahad expresses His oneness in terms of the negative attributes1 (The attributes of Jalâl). For example, the word Ahad in the Surah al-Ihlas2 bears the meaning of Allah has no sharik and similar, he is One and single. Therefore, Surah al-Ihlas is also named as Surah al-Tawhîd. Al-Wâhid expresses His oneness in terms of the positive attributes3 of Jamâl, and His being one and single with His attributes.
1 As-Sifât As-Salbiyya: The negative attributes denote the negation of defects, deficiencies, needs or restrictions that belongs to the created beings and is impossible to be associated with Allah. As-Sifât As-Salbiyya are many. For example, uncreated, unbegotten, undying, etc.
The Dhati (Essential) attributes like Wahdâniyyah, Qidam (Eternity in the past), Baqâ (Being Eternal), Mukhalafa lil-Hawadith (Non-resemblance to the creatures), Qiyam bi Nafsihi (Self-existence), Wujûd are also considered as As-Sifât As-Salbiyya since by their meanings they negate defects, deficiencies, needs or restrictions that belongs to the created beings and is impossible to be associated with Allah. For example, by its meaning, Wahdâniyyah negates the shariks, and Qidam negates the transitoriness.
2 (Qur’an 112)
3 As-Sifât As- Thûbûtiyah: The positive attributes which are befitting Allah and pertaining to His Essence. Hayy (Life), ‘Ilm (Knowledge), Sam’ (Hearing), Basar (Seeing), Qoudrah (Power), Irâdah (Will), Kalam (Speech).
The Dhati (Essential) attributes are the attributes that belong to the essence of Allah only and that are impossible and impermissible to be attributed to any of His creatures.
As for the Thûbûti (positive) attributes, although there is a similarity in naming, they are in no way similar to the attributes of creatures.