Mobile devices are pervasively used for a variety of tasks, including the processing of sensitive data in mobile apps. While in most cases access to this data is legitimate, malware often targets sensitive data and even benign apps collect more data than necessary for their task. Therefore, researchers have proposed several frameworks to detect and track the use of sensitive data in apps, so as to disclose and prevent unauthorized access and data leakage. Unfortunately, a review of the literature reveals a lack of consensus on what sensitive data is in the context of technical frameworks like Android. Authors either provide an intuitive definition or an ad-hoc definition, derive their definition from the Android permission model, or rely on previous research papers which do or do not give a definition of sensitive data. In this paper, we provide an overview of existing definitions of sensitive data in literature and legal frameworks. We further provide a sound definition of sensitive data derived from the definition of personal data of several legal frameworks. To help the scientific community further advance in this field, we publicly provide a list of sensitive sources from the Android framework, thus starting a community project leading to a complete list of sensitive API methods across different frameworks and programming languages.