The BCI ontology specifies a foundational metadata model set for real-world multimodal Brain-Computer Interaction (BCI) data capture activities. Its structure depicts a conceptual framework that BCI applications can extend and use in their implementations, to define core concepts that capture a relevant and interoperable metadata vocabulary. This ontology is aligned to the Semantic Sensor Network Ontology (SSN): a domain-independent and end-to-end model for sensor/actuator applications. Hence, its structure has been normalized to assist its use in conjunction with other ontologies or linked data resources to specify any particular definitions (such as units of measurement, time and time series, and location and mobility), that specialized applications in the BCI domain might need. Its spec provides general alignment data modeling guidelines for core concepts, to help BCI applications in their design.