A configuration block that queries information about existing infrastructure (created inside or outside of Terraform) without managing or modifying it. Syntax: data "type" "local_name" { filter arguments }.
In Terraform Associate it's distinguished from a resource: a resource creates/manages infrastructure; a data source only reads. Question: reference the ID of a VPC created manually in the console to use it in a new subnet → data source, not resource.
Terraform Data Source is a concept that can show up on the exam. Check the full guide to see it in context.