الْبِدْعَةُ: مَا كَانَ مِنَ الْعِبَادَاتِ أَوِ الْاِعْتِقَادَاتِ أَوِ الْأَقْوَالِ لَيْسَ عَلَيْهِ دَلِيلَ مِنَ الْكِتَابِ وَالسُّنَّةِ
Whatever is from the matters of worship or from matters of creed and beliefs or from sayings and it does not have a proof from the Book and the Sunnah.
(From Ithaaful Qaari of Shaykh Saalih al Fawzaan)
For a longer explanation in pdf format CLICK HERE
Islam: The Definitions 