An AWS access key is a set of security credentials consisting of an access key ID and a secret access key. These credentials are used to authenticate and authorize programmatic requests to Amazon Web Services (AWS) resources via AWS APIs, CLI, or SDKs. Access keys are associated with AWS Identity and Access Management (IAM) users or roles and should be kept secure, as they provide the same permissions as the associated user or role. If compromised, unauthorized users could gain access to AWS resources. Best practices include rotating access keys regularly, using IAM roles where possible, and never embedding keys in publicly accessible code or repositories.