Prosopagnosia, or "face blindness," is a difficulty recognizing faces. It can be developmental (lifelong, with no apparent cause) or acquired (following brain injury). Estimates of developmental prosopagnosia run as high as ~2.5% of the population in some samples, though severity varies widely.
Prosopagnosia is distinct from difficulty imagining faces, which is more often associated with visual aphantasia. Face recognition (perceiving a face when you see it) and face imagery (generating a mental picture of a face) use overlapping but separable neural systems. You can have aphantasia with normal face recognition; you can have prosopagnosia without aphantasia; or both can co-occur. People with prosopagnosia commonly rely on non-face cues—voice, gait, hair, clothing, or context—to identify people they know. While Imagination Index measures voluntary imagery vividness rather than face perception, the two questions often come up together for users investigating their cognitive profile.