A user persona is a fictional character created to represent a user type that might use a site, brand, or product in a similar way. Personas are constructed from data collected from user research and segmentation to help design teams understand users' needs, experiences, behaviors and goals.
Personas prevent self-referential design ("I like it, so users will too"). They anchor design decisions in empathy for specific archetypes. However, relying on stereotypes rather than data can lead to misguided product strategies.
Personas are hypotheses, not facts. We use them as a starting point, but we validate them continuously against real analytics. If the data contradicts the persona, the persona dies. We prefer "Jobs to be Done" (JTBD) over demographic personas.