Lists can be defined in a locals block. Let's say you have the following files on your Terraform server.

├── modules.tf
├── locals.tf
├── outputs.tf
├── providers.tf
├── terraform.tfstate
├── variables.tf
├── child (directory, child module)
│   ├── data.tf
│   ├── outputs.tf
│   ├── resources.tf

And let's say locals.tf file has the following.

locals {
  environments = ["dev", "stage", "prod"]
}

And let's say outputs.tf in the same directory as your main root module (main.tf) has the following.

output "my-environments" {
  value = local.environments
}

The terraform output command should return the following.

my-environments = [
  "dev",
  "stage",
  "prod",
]