IAM (Identity and Access Management)
AWS's service for controlling who, and what, can access which resources, forming the security foundation of every account and every migration.
Definition
IAM (Identity and Access Management) is the AWS service that controls authentication and authorisation across an account: which users, roles, and services can perform which actions on which resources. Misconfigured IAM is one of the most common root causes of both security incidents and operational headaches during migration, since overly broad permissions granted for expediency during a project rarely get tightened up afterward.
In Practice
A migration team given full administrative access to move quickly should have that access scoped down to least-privilege roles once the project is complete, not left in place indefinitely.