What is the ruling on one who forgot to pay Zakat al-Fitr, and what should he do if he wishes to pay it after Eid, and what is it called then?

It is obligatory to give Zakat al-Fitr before the Eid prayer. It is a duty and does not lapse. For whoever intentionally delays it past the prayer, it is not accepted from him as Zakat, but rather becomes a charity among other charities, as the Prophet (peace be upon him) said: "Whoever pays it before the prayer, it is an accepted Zakat; and whoever pays it after the prayer, it is a charity among other charities." However, whoever forgets it is excused, and it will be accepted from him as Zakat if he pays it after the prayer.