I don't know the true definition of the term "natural game". But I can give some examples. Ther term "natural game", came a lot during discussion about Ashraful. So lets take him as an example:
Examples of "natural game": All of Ashraful's innings where he scored 50+.
Example of "un-natural game": All of Ashraful's innings where he failed to score double digit.
Shujoger Obhabey Chorittobaan