Final answer:
In the provided sentence, 'ferocious' is an adjective that describes the noun 'tiger', adding to the description of the tiger's nature.
Step-by-step explanation:
The word 'ferocious' in the sentence 'The ferocious tiger rolled over onto his back and nipped lightly at her cubs as they played around her' is an adjective. An adjective is a part of speech that modifies or describes a noun. In this context, 'ferocious' is used to describe the noun 'tiger', giving the reader a better understanding of the tiger's fierce or savage nature. However, the way 'ferocious' is used here could also be seen as ironic since the tiger's actions are gentle with her cubs.