Conceiving Software Architecture Description Languages (ADLs) has been the subject of intensive research in the last two decades resulting in the definition of several ADLs for describing static and dynamic software architectures of (often large) single systems. However, none of these ADLs has the expressive power to formally describe the software architecture of a System-of-Systems (SoS). For addressing this research challenge, this paper proposes a novel ADL, named SosADL, specially conceived to formally describe the software architecture of SoSs. It embodies SoS architectural concepts while providing a formal language supporting automated analysis. Its toolset is outlined. The use of SosADL is illustrated through an extract of a case study.