BIDAH linguistically means innovation. This word in the islamic usage means to innovate something new into the religion (Islaam).
The prophet Muhhamed (salllahualihiwasallam) is reported to have said : "He who innovates or gives protection to an innovator, there is a curse of Allah and that of His angels and that of the whole humanity upon him. (Translation of Sahih Muslim, Book 9)
Abd allah ibn Umar (ralliallahu anhu) said : "Every innovation is misguidance, even if the people see it as something good.''
Sufyan at Tawri mentions: "Innovation is more beloved to Iblees than sin, since a sin may be repented from but innovation is not repented from" and "Whoever listens to an innovator has left the protection of Allaah and is entrusted with the innovation."
Likewise, Shaikh Saalih Aal ash-Shaikh, Minister of Islamic Affairs of Saudi Arabia, stated:
Muslims are of two groups: Salafis and Khalafis. As for the Salafis, they are the followers of As-Salaf-us-Saalih (the 'pious predecessors' from the first three generations of Muslims). And as for the Khalafis, they are the followers of the understanding of the Khalaf and they are also called Innovators – since everyone who is not pleased and satisfied with the path of the Salaf-us-Saalih, in knowledge and action, understanding and fiqh, then he is a khalafi, an innovator.
The criterion that qualifies a particular action as bid`ah is debated amongst Sunni scholars. Some argue for a definition that entails anything not specifically performed or confirmed by the Prophet. This is essentially the stance taken by Muhamamd ibn al-Uthaymeen as quoted above. Arguing for this position Muhammad ibn Salih al-Munajjid, a famous Saudi Arabian scholar, declares:
[H]ow can there be any such thing as bid’ah hasanah (“good innovation”) when the Prophet (peace and blessings of Allaah be upon him) said: “Every bid’ah is a going astray and every going astray is in Hell-fire”. So, if anyone says that there is such a thing as bid’ah hasanah, he can only be insisting on going against the Messenger of Allaah (peace and blessings of Allaah be upon him).... It (referring to a spontaneous form of dhikr in the prayer by a Companion recorded in the hadith literature) was not even considered to have been a correct action until after the Prophet (peace and blessings of Allaah be upon him) had approved it, and not before. But how on earth could this innovator obtain the approval of the Prophet (peace and blessings of Allaah be upon him) after he has passed away?"
—Muhammad ibn Salih al-Munajjid, Islam-QA: "There is no such thing as bid'ah hasana in Islam"