Final answer:
The word 'ferocious' in the given sentence is an adjective, describing the noun 'tiger'.
Step-by-step explanation:
In the sentence provided, 'The ferocious tiger rolled over onto his back and nipped lightly at her cubs as they played around her,' the word 'ferocious' is an adjective. An adjective is used to modify a noun, which in this case is 'tiger,' giving more information about the tiger's fierce and aggressive nature.