What is innovation (bid'ah), and is praying more than eight rak'ahs in Tarawih prayer considered an innovation?

Linguistically, innovation (bid‘ah) is the initiation of something without a prior example. From this derives the Almighty's saying: ﴿Originator of the heavens and the earth﴾.

Religiously, it is what has been introduced into the religion of Allah, having no general or specific basis to support it, such as innovative dhikr using the singular name (Allah… Allah…) or the pronoun (He… He…).

As for increasing the number of rak‘ahs for Tarawih prayer beyond eleven or thirteen rak‘ahs, it is permissible, due to the Prophet, peace and blessings be upon him, saying: "Night prayers are two by two." However, it is contrary to the Sunnah.