Proprietary software solutions are extensively deployed through multiple platforms including desktop, web and mobile devices. Such systems are in many cases closed, thus making it difficult for software developers to expand and create additional features to these systems. Recent research in the field of web and mobile technologies indicates that open systems are more successful than those based on proprietary technologies. In this paper, we investigate and analyze existing research related to the notion of open architectures. We conducted a literature survey of the state of the art projects and definitions related to this concept. Our efforts have resulted in a refined definition of the term followed by a conceptual model of an open architecture contextualized within the field of web and mobile software. The model primarily relies on four key characteristics, namely flexibility, evolvability, customizability and extensibility. Furthermore, we present a validation plan based on the GQM approach that further instantiates these characteristics into a set of properties that can serve as metrics.