I have see references to AWS master account in different projects documentation like for example in awslabs/aws-deployment-framework:
CloudTrail configured in the us-east-1 region within the AWS Organizations Master AWS Account.
What is a master account? I don't see it mentioned in the AWS Organizations terminology and concepts
From https://docs.aws.amazon.com/organizations/latest/userguide/orgs_best-practices_mgmt-acct.html
AWS Organizations is changing the name of the “master account” to “management account”. This is a name change only, and there is no change in functionality. You might continue to see a few instances of the old term while we complete the work to transition to the newer term. If you see one we missed, please use the Feedback link at the top of that page to let us know.
So master account is the old name for the management account.