Definition: the SERVQUAL Model is an empiric model by Zeithaml, Parasuraman and Berry to compare service quality performance with customer service quality needs.
It is used to do a gap analysis of an organization's service quality performance against the service quality needs of its customers.
That's why it's also called the GAP model.
It takes into account the perceptions of customers of the relative importance of service attributes. This allows an organization to prioritize.
The main aspects of service quality are RATER: Reliability, Assurance, Tangibles, Empathy, and Responsiveness.