The development of immersive virtual reality (IVR) hardware and software has accelerated in recent years. The conceptual vocabulary has, however, not received the same amount of attention, especially in the context of collaborative learning settings. Existing concepts such as immersion, presence and interactivity focus predominantly on the individual user's experience, neglecting the social and collaborative dimensions of learning supported by IVR. This limitation is particularly evident in the context of collaborative 360-degree virtual reality (360VR), in which learners must interact with each other to understand the recorded activity rather than interact directly with the environment itself. This paper challenges the current conceptualisations of IVR learning processes and proposes a new conceptual framework: Activity-based collaborative virtual reality (ABC-VR). ABC-VR builds on activity theory and is aligned with a dual sense of activity -- the 'original' activity recorded with 360-degree video cameras and the activity produced by learners collaboratively and interactively immersing themselves in an ABC-VR session. In this dual sense of activity, ABC-VR is oriented to activities and the ways learners inhabit activities from educational, social and work practices, where actions and operations occur in a situated context shaped by the learners' embodied actions and use of material resources. In ABC-VR, learners can question each other and imagine future actions -- but they can also figuratively reason, imagine and reflect with the participants in the original activity. Through this paper, we unfold a conceptualisation of collaborative 360VR that can support the design of pedagogical activities and future research on IVR in collaborative learning activities.