Final answer:
Atypical nevi are unusual moles distinct for having asymmetric shape, irregular borders, color variations, size greater than 6mm, and changes over time. Additional signs indicative of nodular melanoma include elevation, firmness and growth.
Step-by-step explanation:
Atypical nevi are unusual-looking moles that have irregular features under clinical examination. They are known for characteristics often summarized by the acronym "ABCDE": Asymmetry (the two sides do not match), Borders (the edges are irregular in shape), Color variation (shades of brown or black), large Diameter (more than 6 mm), and Evolving over time (changes in its shape).
Further attributes for the more serious form, nodular melanoma, include: Elevated (raised on the skin surface), Firm to the touch, and Growing (increasing in size). Moles which are benign accumulations of melanocytes contrast against melanomas, and these structures are distributed across the skin's landscape.