Proof by subtraction
A word that occurs only once can be defined without any point of comparison, when a series of negations surrounds it: each negation removes a possible meaning, and what remains after all the subtractions is its meaning.
The founding case is ٱلصَّمَدُ, which occurs only once in the Qur'an (112:2). No other occurrence allows comparison. But the verses that follow are three negations: « لَمْ يَلِدْ » (He begets not), « وَلَمْ يُولَدْ » (He is not begotten), « وَلَمْ يَكُن لَّهُۥ كُفُوًا أَحَدٌۢ » (none is His counterpart). The first rules out that anything comes out of Him, the second that He came out of anything, the third that anything stands facing Him. What remains is a being into which nothing enters and out of which nothing comes: "the Impenetrable".
The formula of oneness, « لَآ إِلَٰهَ إِلَّا هُوَ », rests on the same mechanism: لَا denies everything, إِلَّا isolates the exception, هُوَ receives what remains. This mechanism is detailed in “No deity but Him: three pieces, one mechanism”.
The rule is a tool: before declaring a rare word undetermined, one checks whether it is framed by negations that delimit its meaning.
In the text
ٱللَّهُ ٱلصَّمَدُ لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُن لَّهُۥ كُفُوًا أَحَدٌۢ