APA Dictionary of Psychology

a deficiency in the ability to function. It can occur at many levels, from the cellular (e.g., problems with the function of neurotransmitters and receptors) to the executive (e.g., deficits in memory). Functional deficits are often a symptom of a birth defect, an injury (e.g., traumatic brain injury), or disease (e.g., Parkinson’s disease).