`variable` block — input parameter

variable "region" {
  type        = string
  default     = "eu-west-2"
  description = "AWS region for deployment"
  
  validation {
    condition     = contains(["eu-west-2", "us-east-1"], var.region)
    error_message = "Region must be eu-west-2 or us-east-1."
  }
}

Variables let modules accept input. You reference them as var.region. Variables can have types (string, number, bool, list, map, object), defaults, descriptions, and validation rules.

Values are passed in via terraform.tfvars, -var flags, environment variables (TF_VAR_region), or defaults.