The reusable, parameterised version of a prompt with slots for the variable parts. Treating prompts as artefacts to be versioned, reviewed and tested, rather than strings pasted into code, is the difference between a demonstration and something maintainable.
Technical view
Should be stored outside application code, versioned, and bound to an evaluation set so changes can be tested. Variable interpolation of untrusted content requires the same discipline as any other injection-prone substitution, including escaping and clear delimitation. Template versions must be recorded against production traffic so behaviour can be attributed, and rollout should follow the same canary discipline as a code change.
Related
Part of AI Terminology Index, section: Controlling and Extending Behaviour at Run Time.