الخالص · al-khāliṣ · pure, purely for

• Met in: V1 p2 ¶1: «وما سوى الخالص لوجه الله من العلم والعمل عند الناقد البصير رد»
• Dictionary form: خَالِص (corpus lemma, 2 times in the Quran; feminine خَالِصَة ×5)
• Pattern: فَاعِل (fāʿil), the "doer" form: that which is pure. The same root gives إِخْلَاص (ikhlāṣ, sincerity; the ifʿāl pattern, like إحياء) and مُخْلِص (a sincere person).
• Meaning here: Knowledge and action done purely for the sake of Allah: everything else is "rejected".

Ibn Faris, Maqāyīs al-Lugha, root خلص, vol 2, p 207

الخاء واللام والصاد أصل واحد مطرد، وهو تنقية الشيء وتهذيبه

One origin running through all its words: cleansing a thing and refining it.

Urdu bridge

خالص (pure, unmixed: خالص دودھ) and اخلاص (sincerity) are everyday Urdu. (to be confirmed)

Quran

31 words from this root (Quranic Arabic Corpus). Most common: مُخْلِص ×11, مُخْلَص ×9, خَالِصَة ×5.

أَلَا لِلَّهِ ٱلدِّينُ ٱلْخَالِصُ وَٱلَّذِينَ ٱتَّخَذُوا۟ مِن دُونِهِۦٓ أَوْلِيَآءَ مَا نَعْبُدُهُمْ إِلَّا لِيُقَرِّبُونَآ إِلَى ٱللَّهِ زُلْفَىٰٓ إِنَّ ٱللَّهَ يَحْكُمُ بَيْنَهُمْ فِى مَا هُمْ فِيهِ يَخْتَلِفُونَ إِنَّ ٱللَّهَ لَا يَهْدِى مَنْ هُوَ كَٰذِبٌ كَفَّارٌ

Unquestionably, for Allah is the pure religion. And those who take protectors besides Him [say], "We only worship them that they may bring us nearer to Allah in position." Indeed, Allah will judge between them concerning that over which they differ. Indeed, Allah does not guide he who is a liar and [confirmed] disbeliever.

Surah Az-Zumar (39), ayah 3 · Saheeh International

Why this ayah: The very word الخالص: pure religion belongs to Allah.

In the Ihya

الخالص: 33 times; الإخلاص: 171 times. The Book of Intention, Sincerity and Truthfulness (Book 37) is devoted to it.