This paper introduces the Mimosa language, a programming language for the design and implementation of asynchronous reactive systems, describing them as a collection of time-triggered processes which communicate through FIFO buffers. Syntactically, Mimosa builds upon the Lustre data-flow language, augmenting it with a new semantics to allow for the expression of side-effectful computations, and extending it with an asynchronous coordination layer which orchestrates the communication between processes. A formal semantics is given to both the process and coordination layer through a textual and graphical rewriting calculus, respectively, and a prototype interpreter for simulation is provided.