Definition of Bid'ah...
Imaam ash-Shaatibee (rahima-hullaah) mentions:
"Linguistically bid'ah (innovation) means 'a newly invented matter'.
The Sharee'ah definition of bid'ah is: "A newly invented way [beliefs or action] in the religion, in imitation of the Sharee'ah (prescribed Law), by which nearness to Allaah is sought, [but] not being supported by any authentic proof - neither in its foundations, nor in the manner in which it is performed".
al-I'tisaam of ash-Shaatibee (1/37)