A plugin that lets Terraform interact with a specific API (AWS, Azure, GCP, Kubernetes, etc.). Each provider exposes resources and data sources. It's declared in the terraform.required_providers block.
Fundamental in Terraform Associate. Without a configured provider, Terraform can't create resources. Providers are downloaded with terraform init. Question: what do you need to manage Azure resources with Terraform? The azurerm provider configured with credentials.
Terraform Provider is a concept that can show up on the exam. Check the full guide to see it in context.