Defining the core features of an MVP involves the following steps: 1. Identify the problem: Understand the specific problem your target audience is facing. 2. Define the solution: Outline how your product will solve this problem. 3. List out essential features: Identify the features that are absolutely necessary to solve the problem. 4. Prioritize these features: Use techniques like MoSCoW (Must-have, Should-have, Could-have, and Won't-have) to prioritize features.