What is the definition of zina (unlawful sexual intercourse) to which the hadd (prescribed punishment) applies in Islam? Is fondling an unfamiliar woman considered zina? Is the hadith "Give tidings to the zani (fornicator/adulterer) of poverty, even if after a while" authentic, and what kind of zina is referred to in it?

You must initiate sincere repentance from this sin (adultery), regret it, and resolve not to return to it, for sincere repentance erases what came before it. Scholars have defined adultery that necessitates a prescribed punishment as: a responsible Muslim's sexual intercourse with a human being, over whom he has no ownership or semblance of ownership, intentionally, and it occurs by the meeting of the two circumcised parts and the insertion of the glans. Furthermore, the ill effects of sins are among the causes of poverty, illness, and anxiety. We advise you to fear Allah, maintain prayers with the congregation, keep company with the righteous, and increase in good deeds.