Software architecting is a knowledge-intensive activity. However, obtaining and evaluating the quality of relevant and reusable knowledge (and ensuring that this knowledge is up-to-date) requires significant effort. In this paper, we explore how online developer communities (e.g., Stack Overflow), traditionally used by developers to solve coding problems, can help solve architectural problems. We develop an ontology that covers architectural knowledge concepts in Stack Overflow. The ontology provides a description of architecture-relevant information to represent and structure architectural knowledge in Stack Overflow. The ontology is empirically grounded through qualitative analyses of different Stack Overflow posts, as well as inter-coder reliability tests. Our results show that the architecture knowledge ontology in Stack Overflow captures architecture-relevant information and supports achieving practitioners' requirements and concerns.