way / practice / established custom
Feminine noun. Plural: سُنَنٌ (sunan). In the Quran, سُنَّةُ اللَّهِ means God's unchanging patterns/laws in history and nature. Also refers to the Prophet's practice.
In the Quran, sunnatu Allāh refers to God's unchanging laws governing nations - those who are righteous prosper, those who transgress are destroyed. A divine pattern that never changes.