Sinus Meridiani is a feature name for an albedo feature on Mars stretching east-west just south of that planet's equator. It was named by the French astronomer Camille Flammarion in the late 1870s. (wikipedia entry)
A albedo feature is an area which shows a contrast in brightness or darkness (albedo) with adjacent areas. On Mars they are named after names from classical mythology assigned by Schiaparelli and Antoniadi.