Creativity Support Tools (CSTs) play a fundamental role in the study of creativity in Human-Computer Interaction (HCI). Even so, there is no consensus definition of the term 'CST' in HCI, and in most studies, CSTs have been construed as oneoff exploratory prototypes, typically built by the researchers themselves. This makes it difficult to clearly demarcate CST research, but also to compare findings across studies, which impedes advancement in digital creativity as a growing field of research. Based on a literature review of 143 papers from the ACM Digital Library (1999-2018), we contribute a first overview of the key characteristics of CSTs developed by the HCI community. Moreover, we propose a tentative definition of a CST to help strengthen knowledge sharing across CST studies. We end by discussing our study's implications for future HCI research on CSTs and digital creativity. CCS CONCEPTS • Human-centered computing → Interaction design theory, concepts and paradigms; HCI theory, concepts and models; Interactive systems and tools.