The mesosystem is a component of the ecological systems theory developed by Urie Bronfenbrenner in the 1970s. It proposes that children don't develop only by influence from their close familial environment - surrounding environments are influential on the development of the child as well.
There are five environmental systems: microsystem (immediate environment like family, school, neighboorhood, and peers), mesosystem (interaction between two microsystems), exosystem (indirect but prominant influences like parents' jobs and the school system), macrosystem (cultural influences like customs, beliefs, and government), and chronosystem (transitional influences over the lifespan, for example growing up during World War II or the civil rights movement). The mesosystem affects children directly and are interactions between two microsystems. Mesosystems can be long lasting (such as their family's involvement in their neighboorhood throughout the years) or a one time only event (like a parent chaperoning a school dance).