In this paper, we present a comprehensive ontology of human cognition and abilities, designed as a formal framework for digital twins. This model enables digital twins of humans to replicate aspects of human cognition, while cognitive digital twins enhance cyber-physical systems with human-like reasoning and intelligence. The cognition meta-model results from the integration of multiple perspectives on cognition in Neuropsychology, Education Sciences, Engineering Sciences, Cognitive Informatics, and Cognitive Architectures, including indirectly also perspectives of Cognitive Sciences and Artificial Intelligence. Our Human Ontology (HUMO) is an extension of the SOMA (Socio-physical Model of Activities) ontology, which serves as a basis to design cases for cognitive robot - human collaboration. It aims at being exploited by digital twins in their representation of the world and the entity they twin.