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

the open ocean

(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