Electrical power quality (PQ) data is a ubiquitous type of engineering data quantified as a set of PQ parameters, such as frequency, flicker, and harmonics, obtained from electricity transmission and distribution systems. For PQ problems detected as deviations from nominal power system behavior negatively affect the workings of governmental and industrial institutions, as well as the electronic equipment and the psychology of individual humans, several systems of varying complexities to measure and monitor PQ parameters have been developed in order to detect the problems and take the necessary countermeasures. Yet, the domain of PQ data still suffers from the lack of a common vocabulary to be shared by the involved parties. In this paper, a novel domain ontology for electrical PQ, called PQONT, is presented to fulfill this need. Relevant standards and regulations have been utilized extensively during the ontology building process. The proposed ontology also supports linguistic applications by providing a number of linguistic properties applicable to all of its concepts. This facility of the ontology is employed in a multilingual natural language based interface for PQ data collected through a nationwide PQ measurement system so that the data can be queried in a flexible way. The implementation details of the interface is presented with a couple of query examples to illustrate the contribution of the ontology to a real application. The future work includes the extension of PQONT to a more general power system ontology so that it can contribute to a wider range of applications in the power system domain. (C) 2009 Elsevier Ltd. All rights reserved.