Final answer:
Fungi have cell walls made of chitin and are heterotrophic organisms, distinct from plant and animal kingdoms.
Step-by-step explanation:
The unique characteristic that is specific to fungi and not found in plants or animals is that their cell walls contain chitin rather than cellulose. Unlike plants that synthesize their own food through photosynthesis, fungi are heterotrophs; they absorb their nutrients from other organisms. Fungi have a significant and distinct evolutionary line and are classified in their own kingdom because of their unique features and functions.