Mashups can be seen as the result of software composition applied to the Web. One of the characteristics of mashup development is the heterogeneity of its building components in terms of logical layering (e.g., user interface, application logic, and data), access method (e.g., REST, SOAP), and composition technique (e.g., scraping vs. clipping, synchronous vs. asynchronous interaction, discrete vs. streaming). This poses a challenge towards the design of mashup tools aiming at lowering the barriers of mashup development, as this heterogeneity needs to be abstracted. In this paper, we address this challenge by proposing a new JSON-based domain-specific language for describing heterogeneous mashup components, called the Mashup Component Description Language (MCDL). MCDL lies at the core of a meta-model for mashup component modeling, and can be used for component discovery and classification but also for user-centric mashup development as it decouples the interface of a mashup component from its underlying implementation technologies.