chief in size or importance
a main road
the main problem is one of resources
a principal pipe carrying water or gas to buildings, or taking sewage from them
a faulty gas main
(in the game of hazard) a number (5, 6, 7, 8, or 9) called by a player before dice are thrown
a match between gamecocks
a river of south-western Germany which rises in northern Bavaria and flows 500 km (310 miles) westwards, through Frankfurt, to meet the River Rhine at Mainz