Anthropomorphism plays a pivotal role in robot design and human-robot interaction, significantly shaping user perceptions and behaviors. Despite a growing body of research, the current understanding of robot anthropomorphism remains fragmented, with limited differentiation across various concepts, research lenses, research methods, and theoretical contributions. This ambiguity hinders the effective design of anthropomorphic features that could enhance human-robot interactions and foster broader adoption of robots. To address these issues, this study conducted a systematic literature review, analyzing 431 previous studies to explore the conceptualization of robot anthropomorphism, along with design-oriented, user-oriented perspectives, and the relationships between the two dimensions. In so doing, this study contributes to the literature by offering a comprehensive understanding of how anthropomorphism has been studied in literature and proposing a research agenda to address unresolved challenges.