What Do We Mean by ‘Replication’ and ‘Validation’ in Genome-Wide Association Studies?

It has been acknowledged that results from genome-wide association studies need to be reproduced in further independent samples. This includes both aspects of replication and validation. Although often used interchangeably, both of these terms have a specific unique meaning and are not synonyms. Based on a number of applications from the literature producing a certain amount of confusion, our aim therefore is to clarify the definition of 'replication' and 'validation' and propose standards of their usage.