«واصطلاحًا: هو تقدير الفرع بالأصل في الحكم والعلة»
Technically, qiyas measures a subsidiary case against an established case with regard to ruling and effective cause.
The asl (original case) has an established ruling; the far‘ (new case) is the case being assessed. The hukm is the legal ruling and the ‘illah is the operative characteristic that connects the ruling to the case.
For example, the prohibition of wine and its intoxicating property can guide analysis of another intoxicant. The similarity must lie in the legally relevant cause—not its name, color, or container.