A device for arriving at legal rulings on situations about which revelation is silent, by applying an established ruling on a given problem to a new problem on the basis of a common factor that makes the two problems similar.
For example, the sources of Islamic law explicitly prohibit grape wine, but say nothing about intoxicating drinks in general. Since intoxicating drinks share a relevant common feature with grape wine (namely that they both produce intoxication), the prohibition of grape wine is extended, by analogy, to intoxicating drinks in general.