Wikidata is one of the most successful Semantic Web projects. Its underlying Wikibase data model departs from RDF with the inclusion of several features like qualifiers and references, built-in datatypes, etc. Those features are serialized to RDF for content negotiation, RDF dumps and in the SPARQL endpoint. Wikidata adopted the entity schemas namespace using the ShEx language to describe and validate the RDF serialization of Wikidata entities. In this paper we propose WShEx, a language inspired by ShEx that directly supports the Wikibase data model and can be used to describe and validate Wikibase entities. The paper presents the abstract syntax and formal semantics of the WShEx language.