فاحشة
fāḥisha
Shameful and indecent conduct, and especially unlawful sexual relations, which the Quran forbids.
What it means
God forbids “shameful deeds, whether open or hidden” (7:33), and prayer “prevents shameful and wrong deeds” (29:45). For adultery the Quran requires proof by four eyewitnesses (4:15, 24:4) and punishes the one who accuses without proof with eighty lashes (24:4). Repentance and amendment are accepted (24:5, 4:16).
What the sources say
The verses on punishment in 4:15–16 and 24:2 are read by jurists as stages of the law, and the very high standard of proof means that in practice conviction on witnesses alone is rare. The Prophet told people to conceal what they could, and to avoid suspicion.