The Unique, The Single, The Only One.
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al-Waahid (in Arabic: ٱلْوَٰحِدُ), the One and only God. There is no alternative. He is the sole originator from which all has arisen. He has no partners or relationships. None is equal to Him.
From the root waw-ha-dal (و ح د), which has the following classical Arabic connotations: to be one, to be made one, to be called one, to be unique, singular, to be one apart from others.
The oneness of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ:
Al-Wahid and Al-Ahad are two names which describe tawhid (the oneness) of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. This is what characterizes our deen and differentiates Islam from other religions. That's why the first step to converting to Islam is to recite the shahada. We testify, la ilaha illa allah, "there is no god but Allah", muhammadur rasul-ullah, "and Muhammad is the Messenger of Allah." This core belief is shared among all Muslims and is the first pillar of Islam. These two names of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ further emphasize this idea of Allah's oneness.
The word Waahid in Arabic means 'one.' For example, someone who is Waheed uz-zaman is said to be a unique person of that era (time). So it's used in a relative sense, but as a name of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ it is absolute. He is unique in all aspects, the only one who embodies perfection. He has no equal. Throughout our guide on the 99 Names of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ we discuss and encourage ways in which we can share in the divine attributes, to be merciful, giving, caring, and a friend to others. But these are human characterizations. It's important to realize that no one comes close to sharing in His attributes or actions. He is alone and unique in His essence. As Imam Zarruq has said, "He is One in His Attributes: He resembles nothing and nothing resembles Him."
Imam al-Ghazali also mentions how Al-Wahid refers to the one who can neither be divided nor duplicated. This quality of being indivisible also makes Him one since He's not separated into parts. For being separated into parts makes the previous part dependent on the other to be considered whole. And Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al-Ghaniyy, The Self-Sufficient - free from dependence on anyone or anything.
Qul mar Rabbus samaawaati wal ard; qulillaah; qul afattakhaztum min dooniheee awliyaaa'a laa yamlikoona li anfusihim naf'anw wa laa darraa; qul hal yastawil a'maa wal baseeru am hal tastawiz zulumaatu wannoor; am ja'aloo lillaahi shurakaaa'a khalaqoo kakhalqihee fatashaa bahal khalqu 'alaihim; qulil laahu Khaaliqu kulli shai'inw wa Huwal Waahidul Qahhar
"Say, 'Who is Lord of the heavens and earth?' Say, 'Allah.' Say, “Have you then taken besides Him allies not possessing [even] for themselves any benefit or any harm?' Say, 'Is the blind equivalent to the seeing? Or is darkness equivalent to light? Or have they attributed to Allah partners who created like His creation so that the creation [of each] seemed similar to them?” Say, 'Allah is the Creator of all things, and He is the One, the Prevailing.'" — (Qur'an 13:16)
Law araadal laahu aiyattakhiza waladal lastafaa mimmaa yakhluqu maa yashaaa'; Subhaanahoo Huwal laahul Waahidul Qahhaar
"If Allah had intended to take a son, He could have chosen from what He creates whatever He willed. Exalted is He; He is Allah, the One, the Prevailing." — (Qur'an 39:4)
Qul innamaaa ana munzirunw wa maa min laahin illal laahul Waahidul Qahhaar
"Say, [O Muhammad], “I am only a warner, and there is not any deity except Allah, the One, the Prevailing." — (Qur'an 38:65)
Yawma tubaddalul ardu ghairal ardi wassamaawaatu wa barazoo lillaahil Waahidil Qahhaar
"[It will be] on the Day the earth will be replaced by another earth, and the heavens [as well], and all creatures will come out before Allah, the One, the Prevailing." — (Qur'an 14:48)
Pairing with Al-Qahhar:
As we discussed in the name Al-Qahhar, we can see the pairing of names with Al-Wahid. Al-Qahhar is the "Prevailing One," the one who dominates absolutely. The pairing is meant to instill humility in mankind. Humans tend to arrange themselves in a hierarchy. We see this with countries, governments, companies, tribes, sports teams, etc. As an individual climbs the ladder, they begin to take more control and serve as a leader to the surrounding people. But they can become corrupted by this power and use it for personal gain rather than the greater good. The higher they climb, the less they may think they have no superior to answer to. But Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ reminds us He is the one who dominates all others and does so alone. No matter how great you may think you are, we will all be held for our transgressions.
Qul ayyu shai'in akbaru shahaadatan qulil laahu shaheedum bainee wa bainakum; wa oohiya ilaiya haazal Qur'aanu li unzirakum bihee wa mam balagh; a'innakum latashhadoona anna ma'al laahi aalihatan ukhraa; qul laaa ashhad; qul innamaa Huwa Ilaahunw Waahidunw wa innanee baree'um mimmaa tushrikoon
"Say, “What thing is greatest in testimony?” Say, “Allah is witness between me and you. And this Qur'an was revealed to me that I may warn you thereby and whomever it reaches. Do you [truly] testify that with Allah there are other deities?” Say, “I will not testify [with you].” Say, “Indeed, He is but one God, and indeed, I am free of what you associate [with Him]." — (Qur'an 6:19)
Supplication from the hadith:
Ibn Abbas reported that Allah's Apostle ﷺ used to supplicate during the time of trouble (in these words):
'La Ilaha Illalahul 'Alzimul Halim, La Ilaha Illallahu Rabbul 'Arshil 'Azim, La Ilaha Illallahu Rabbus Samawati Wa Rabbul Ardi Wa Rabbul 'Arshil Karim
"There is no god but Allah, the Great, the Tolerant, there is no god but Allah, the Lord of the Magnificent Throne There is no god but Allah, the Lord of the Heaven and the earth, the Lord of the Edifying Throne." 
Narrated Mu'adh bin Jabal: The Messenger of Allah ﷺ as saying: If anyone's last words are "There is no god but Allah" he will enter Paradise.