(formerly) the mainland of America adjacent to the Caribbean Sea, especially the area between the mouth of the Orinoco River and the Isthmus of Panama.
2.
the Caribbean Sea: the route of the Spanish treasure galleons and a former haunt of pirates.
British Dictionary definitions for Spanish Main
Spanish Main
noun
1.
the mainland of Spanish America, esp the N coast of South America from the Isthmus of Panama to the mouth of the Orinoco River, Venezuela
2.
the Caribbean Sea, the S part of which in colonial times was the route of Spanish treasure galleons and the haunt of pirates