ELK is a reasoner for OWL 2 ontologies that currently supports (most of) the OWL 2 EL ontology language.

ELK is a free and open source reasoner for the lightweight ontology language OWL 2 EL, developed by the University of Oxford and the University of Ulm. It is based on Java and can be controlled using the OWL API, Protégé, the Snow Owl ontology editor, or a basic command line interface. ELK is available under the Apache License 2.0.

ELK is massively parallel and insanely fast. It can classify the SNOMED CT ontology with around 300,000 classes in a few seconds on a modern laptop. It runs on all operating systems that support Java 1.6 or above. ELK is being jointly developed at the University of Oxford (by Markus Krötzsch and František Simančík) and the University of Ulm (by Yevgeny Kazakov, Pavel Klinov, and Peter Skocovsky). Its development is supported by projects  ConDOR (Oxford) and Live Ontologies (Ulm).

More information about ELK can be obtained from the GitHub project page.