Crowdsourcing is an emerging strategy for enterprises to harvest information, labour, expertise and innovation from a wide, anonymous crowd. To fully adopt this strategy, enterprise systems should not only extend existing frameworks with new services and users, but also efficiently integrate crowdsourced activities with, usually more stable, organisational processes. However, an ontological structure for this integration is still missing. Although a few lightweight ontologies have been proposed in the crowdsourcing domain, they seem to focus on clarifying concepts rather than providing a structure for modelling, designing, and developing an enterprise system. The current study fills the gap by building an enterprise ontology of business process crowdsourcing. Our results identified the main business processes, data entities, data attributes, and their hierarchy relationships, which were structured into a lightweight ontology. We then added decision-making relationships and business rules that turn a lightweight into a heavyweight ontology. The built ontology is evaluated by triangulation when comparing with two ontology versions generated by automated tools. The current study is significant as it is the first effort to build an enterprise ontology of crowdsourcing, which provides a blueprint for integrating business process crowdsourcing into enterprise systems.