Terraform variables allow users to write flexible, reusable, and scalable configurations by abstracting values that may change between environments or deployments. Variables enable the passing of dynamic inputs, ensuring configurations are adaptable and maintainable across different scenarios. Terraform enables validations for input variables, which can prevent problems before they occur, ensuring infrastructure integrity, security, and reliability. Variable validation in Terraform is a mechanism to enforce custom rules on input variable values during plan or apply operations. It helps ensure variables meet specific constraints, improving reliability and preventing misconfiguration. With a simple variable definition, users can easily assign and pass values around their configuration. However, without validations, these variables can receive any value, which can cause configuration issues. Validation provides a way to enforce constraints on variable values, such as data type, format, or range, which helps improve the stability and reliability of the infrastructure as code. Terraform offers three types of variable validations: type validation, required validation, and custom validation. Type validation ensures input values conform to expected data types, while required validation makes sure all variables have an assigned value. Custom validation uses a validation block with two properties: condition and error_message. The condition specifies the rule for the variable to be valid, and the error_message provides the output if the condition is not met. Terraform also allows referencing other objects in variable validations, such as variables, locals, data sources, and resources. To avoid unexpected behaviors, it's essential to understand when validation will run during the plan or apply phase. Best practices for using variable validations include writing descriptive error messages, keeping conditions simple and readable, breaking down long validations, validating against live data, and understanding which step validation will run. Terraform management tools like Spacelift make managing Terraform state, complex workflows, policy as code, programmatic configuration, context sharing, drift detection, resource visualization, and more easier.