The return type of a SOSL search is List of List of sObjects.

Sample Code:

List<List<SObject>> searchList = [FIND 'map*' IN ALL FIELDS RETURNING Account (Id, Name), Contact, Opportunity, Lead];
Account [] accounts = ((List<Account>)searchList[0]);
Contact [] contacts = ((List<Contact>)searchList[1]);
Opportunity [] opportunities = ((List<Opportunity>)searchList[2]);
Lead [] leads = ((List<Lead>)searchList[3]);