Definition of Bid'ah

Bid'ah, literally means innovation. In special terms it means carrying out actions which displease Allah ta`ala and his messenger. 

[Tirmizi chapter Il]

Qadi Shawkani writes "In Islam there are two kinds of Bid'ah: Bid'ah Say'iah and Bid'ah Hasana. If a new thing opposes the Qur’an and Sunnah then it is Say'iah, but if it is not against the Shari'ah then it is Hasanah.

(Nayl-ul-Autaar, Qadi Shawkani chapter Salaah Al Taravee).

Imam Nawawi writes that there are certain types of Bid’ah. Two of them are Bid’ah Sayyiah and Bid’ah Hasanah. Bid’ah sayyiah is a Bid’ah that opposes the Qur’an and sunnah and Bidah Hasanah is a Bid’ah that is not against the Qur’an or Sunnah. For example:

To invent the usool (principle) of Hadeeth, usool of Fiqh, usool of Tafsir etc.

[Tahzeeb al Asma wal lughaat word Bid’ah by Imam Nawawi]

Hafidhh ibn Rajjab defines bida to mean new things that have no basis in the Qur'an or Sunnah. If a new practice has evidence from the Qur'an or Sunnah it will not be Bid'ah Shari'ah, but it will be Bid'ah Logaviyya (linguistic). 

(Jaami' Al Uloom Al Hukkam page 252 by Hafidhh ibn Rajjab). 

Hafidhh Asqalani writes if a new thing is against Islam, it will be bad. If it is not against Islam, it will be hasanah (Good). 

(Fathul Bari chap on Taravi by Hafidhh Asqalani).

Hafidhh ibn Taymiyyah writes that bid'ah is always bad, but some scholars say that there are two kinds of bid'ah, that one is good and one is bad. If a new thing has origin in the Qur'an and Sunnah it will be called Bid'ah Logaviyya (verbally) but not Bid'ah in Shari'ah. Only the word bid'ah will be used on the new things. Like, the Qur'an was collected in one book after the Prophet Muhammad [May Allah bless Him and grant Him peace], and the congregational Taravi prayer was started in Sayyidna Umar's time but these two things have an origin in the Sunnah. Therefore, it will be called bid'ah verbally. 

(Iqtidah al Sirat al Mustaqeem chap on Bid'ah by Hafidhh ibn Taymiyya). 

From the above, the conclusion is that if a new thing has been started, and it neither goes against the Qur’an or Sunnah, then it can be declared a ‘good’ innovation. However, if a new act is initiated against the Qur'an and Sunnah, that will be called bad bid'ah, or a reprehensible innovation.