Shaikh Abdul Muhsin Al Badr who is the Muhaddith of Madinah said that the word Sunnah has four meanings:
1) The linguistical meaning which means way or path.
2) The meaning the scholars of Hadeeth apply to it which is the sayings, actions and permissions of the Messenger (Salallahu alaihi wa sallam)
3) The meaning that the scholars of usuul ul fiqh apply to it which is that the Sunnah means what is liked from extra actions which a person if he does it he gets reward but if he doesn’t do it he gets no punishment.
4) Meaning the Creed or Aqeedah. And this is why you have many books during the time of the Salaf were named using the word Sunnah. For example:
i) Usuul Us Sunnah by Imam Ahmad Ibn Hanbal. This book describes the creed of the Salaf
ii) Usuul Us Sunnah by Imam Al Humaidi
iii) As Sunnah by Abdullah Ibn Ahmad Bin Hanbal which is used to describe creed
iv) Kitaab Us Sunnah which is in Sunan Abi Dawuud which describes creed again.
v) Sareehus Sunnah by Imam Ibn Jarir At Tabari.
And other books of that nature which describe the belief of Ahlus Sunnah Wal Jama’ah