The term 'Mamma Mia Quartet' refers to the four members of the Swedish pop group ABBA, which stands for Agnetha, Björn, Benny, and Anni-Frid. The acronym EMS stands for the three main songwriters of ABBA, which are Björn Ulvaeus, Benny Andersson, and Stig Anderson. Together, these four members and three songwriters formed the iconic band ABBA, which is widely known for their hit songs such as 'Dancing Queen' and 'Mamma Mia'.