The short definition, layperson version:

Neuropsychology is a specialized domain of psychology that is focused on the relationships between the phenomena of cognitive and affective functioning, personality and behaviour of the individual, and brain functioning. Cognitive functions include e.g. attention, learning, memory, language, reasoning. Affective functions include emotions and mood. Clinical Neuropsychology is the application of neuropsychology in clinical situations across the life span to understand how these phenomena relate to either normal brain functioning or acquired or developmental brain injury. The practice of Clinical Neuropsychology involves a clinical interview and the use of neuropsychological assessment methods for diagnostic and prognostic purposes. The information obtained can also be used for planning, implementation and evaluation of interventions such as rehabilitation.

The Standing Committee on Clinical Neuropsychology of EFPA