Confusing terms in docs: "AWS Root User" vs "IAM Root User"

The terms "AWS Root User" and "IAM Root User" refer to the same concept. This is the initial user identity that is created when you first sign up for an AWS account. This root user has complete access to all AWS services and resources in the account with permissions that cannot be changed.

Some key characteristics of the root user:

1. It's created automatically when you establish an AWS account
2. You sign in as the root user using the email address and password used to create the account
3. It has complete access to all AWS services and resources in your AWS account
4. It's also referred to as the "account owner" or "account root user"

AWS strongly recommends not using the root user for everyday tasks, even administrative ones. Instead, you should create other types of IAM identities (like IAM users, roles, or use IAM Identity Center) with appropriate permissions for regular activities.

The inconsistent terminology across AWS documentation ("AWS Root User" vs "IAM Root User") is referring to the same concept - the initial user with full privileges created when an AWS account is established. This root user exists outside the standard IAM user hierarchy but is often discussed in IAM documentation because of its relationship to identity and access management.
Sources
Determine your user type - AWS Sign-In
ACCT.02 Restrict use of the root user - AWS Prescriptive Guidance
Compare IAM identities and credentials - AWS Identity and Access Management
IAM Identities - AWS Identity and Access Management

Actually, it is called AWS Account Root User (https://docs.aws.amazon.com/IAM/latest/UserGuide/id_root-user.html). When we create an AWS account, an AWS account root user is created (the one identified with the email address and the password you define when setting up the AWS account). That user has all the permissions for the account. In fact, it is the root user the only one who can perform specific actions (https://docs.aws.amazon.com/IAM/latest/UserGuide/id_root-user.html#root-user-tasks). Thus, the AWS account root user doesn't have any policy and you don't see it in AWS IAM.

Once an account is created, the AWS account root user can create administrative users in AWS IAM. As indicated in the documentation, the AWS account root user is not the same as the administrative user because, as mentioned above, there are specific tasks that only the root user can perform. However, it is a good practice to NOT use the root user for the daily tasks (https://docs.aws.amazon.com/IAM/latest/UserGuide/root-user-best-practices.html).

Find here a link to the documentation: https://docs.aws.amazon.com/signin/latest/userguide/user-types-list.html