Shaykh Al-Fawzaan said, (in his explanation of Imam Ibn Taymiyyah’s book Al Aqeedatul Wasitiyyah), “They are called ‘Ahlu Sunnah’ because of their connection to the Sunnah of the Messenger – sallallahu alayhe wa sallam – preferring this (his Sunnah) above all, [they put it above] other than it from statements and madhabs.
This is in opposition to Ahlu Bidah (the people of innovation). Because indeed they are [named] based on their connection to their bidah or misguidance like the Qadariyyah [who went astray in the issue of Allah’s qadar] and the Murji’ah [who strayed in the issue of iman].
Sometimes they are [named] based on their ugly, evil actions like the Raafidah [extreme Shia seen mostly in Iran] and the Khawaarij [those who wage armed jihad against the ummah and declare Muslims as disbelievers due to their major sins].
[And Ahlu Sunnah Wal Jamaah have connected to their name] the Al-Jamaah. [Jamaah means] in the language a group of people gathered together. What is intended here is those gathered together upon the truth clinging to the Book and the Sunnah. They are the Sahabah and all who follow them perfectly [in creed, statements and actions] even if they are few.
As Ibn Mas’ood – radiAllahu anhu – said, “The Jamaa’ah is what agrees (or conforms) with the truth, even if you are by yourself, in that case you are the Jamaah.””
Ref: markazsunnahleicester.com with slight adaptations by ahlusunnahwaljamaah.com for easier reading and understanding
Posted by Abdul Kareem Ibn Ozzie