Allāh
This article isn't about the God of Islam, which is clearly a monotheistic Abrahamic god. This post is about the use of the term Allāh among pre-Islamic pagans.
The name allāh is found in pre-Islamic pagan inscriptions, there is no doubt about it. Arab Christians from the 1st to 4th centuries used the word al-ilāh (the god) to refer to their monotheistic deity, not allāh. Christians do begin using al-ilāh and allāh interchangeably later on in the 6th century as seen in the poetry of al-Nābigha al-Dhubyānī and ʿAmr ibn al-Ḥārith. The Nabataeans often referred to their main deity Ḏušarē simply as ʾlhʾ (the god) and so allāh being a contraction of al-ilāh used as an epithet that replaced a respective deity’s proper name is the most likely answer. Thus, we can ask a different question. What did the word allāh mean to the pre-Islamic Arabs? Allāh meant the god. The main god, whether that be of the Christians or Pagans. That seems to be the most likely answer; that allāh is a contracted form of the word al-ilāh.
However, Allāh is sometimes used in pagan inscriptions as the name of a particular deity rather than just an epithet/title. In one inscription Allāh and the god Šayʿhaqqawm are both appealed to for abundance and safety. In another inscription, Allāh is paired with Ruḍāw. Here we see allāh clearly being used as a proper name and not just an epithet that implies one deity was superior to others. We even see Allāh being paired with Allāt. So who was Allāh to the pagan Arabs? Allāh seems to be whichever principal deity was being worshipped locally. If this is correct, then in Dedan, it was Ḏu Ɣabat. In the Nabatean context, it was Ḏušarē. In the Meccan context...well it gets a bit more complicated. Allāh is mentioned in pre-Islamic poetry as a creator deity even by apparently pagan poets, which is inline with Islamic tradition. There seems to be an acceptance of the idea of divine creation, both of the cosmos as a whole and of humans specifically by at least some pagan poets, although not always explicitly mentioning Allāh. Safaitic inscriptions also ask Allāh to grant rain, and in poetry as well, or he is credited with having already done so. And although some scholars suggest that Allāh may have represented a remote creator god, pre-Islamic poetry suggests that he was very much involved in human affairs, both by granting favors and by being destructive and threatening. In a poem boasting about a successful raid by his tribe the poet says: "the order of Allāh is irresistible and the wicked are rendered miserable by it." In poetry Allāh seems to be the ultimate origin and allocator of fate.
A lot of pre-Islamic poetry mentions al-ilāh (the god), Raḥmān (the Merciful) and al-rabb (the Lord) as a deity. These are epithets and its not always clear if the poet was pagan or not. There's also the question of the authenticity of these poems since they were written down ~200-300 years after they were first recited. Scribes may have censored pagan themes or altered the wording or straight up made some of these up. Although sometimes we do see mentions of pagan deities such as Allāt and Wadd. It's also important to consider that some of these poets actually later converted to Islam, and of course they continued writing poetry but now with a new monotheistic worldview. Even when it comes to poets who were presumably pagan, many poems were recited to Christian kings such as the poems pronounced by ʿĀmir ibn al-Ṭufayl at the court of Nuʿmān III. He probably would have given his poems a Christian bend to please the Lakhmid king.