وَأَوَّلُ وَقْتِ الْمَغْرِبِ إِذَا غَرَبَتِ الشَّمْسُ، وَآخِرُ وَقْتِهَا مَا لَمْ يَغِبِ الشَّفَقُ
Translation: Maghrib begins at sunset and continues until twilight (shafaq) disappears.
Maghrib starts at sunset. The juristic disagreement concerns which remaining twilight glow is meant by shafaq.