ToFileName

ToFileName["directory","name"] assembles a full file name from a directory name and a file name. 

ToFileName[{dir1,dir2,…},name] assembles a full file name from a hierarchy of directory names. 

ToFileName[{dir1,dir2,…}] assembles a single directory name from a hierarchy of directory names. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.