Glossary
The vocabulary, defined plainly.
What each term means, where it came from, and where it stops being useful.
$curl -fsSL https://lucidtrain.com/install.sh | sh
- What Is the C4 Model?Four levels of zoom on one system. Two of them get maintained; the other two usually do not.
- What Is an Architecture Decision Record?The code shows what was decided. Only the record shows what was rejected, and why.
- What Is Architecture Drift?Nobody decides to let the diagram go stale. It goes stale because nothing in the process notices.
- What Is Diagram as Code?Text in, picture out. Which means the layout engine, not you, decides what it looks like.
- What Is a Sequence Diagram?Time runs down the page. That single convention is what the diagram is for.
- What Is Spec-Driven Development?Review the plan, not the pull request. The plan is a page; the pull request is a thousand lines.
- What Is a Container Diagram?The most useful diagram most teams do not have, and the worst-named one in the field.
- What Is a System Context Diagram?One box for everything you own. The value is entirely in what surrounds it.
- What Is a Component Diagram?Useful for the one container that is genuinely complicated. Drawn far more often than that.
- What Is Agentic Coding?The difference is the feedback loop. An agent can find out it was wrong.
Elsewhere
- System Design TemplatesEach template is a real diagram produced by the same layout engine the app runs, not a picture of one.
- AI System DesignThe diagram can cause the code, not just describe it.
- Harness EngineeringAgent equals model plus harness. The harness is the half you can actually change.
- Coding Engines Lucid Train Can DriveThe binary is already on your machine and already signed in. Nothing is proxied and no credential passes through us.
- Dependency and Agent Supply Chain AuditYour packages are audited by everyone. The programs your agent launches are audited by almost no one.
- Grafana and Observability in a Coding AgentNothing is exported to us. It reads the Grafana you already run, with your credential, on your machine.
- AI Design Mode for Web PagesRunning on the Claude Code or Codex CLI you already have, with your repository open beside it.
- Guides to Choosing Architecture ToolsNo winner is declared, because the right answer depends on constraints a web page cannot see.
- Alternatives to Popular Diagramming and Agent ToolsEach page sorts the field by the reason you are leaving, because that is what decides the answer.
- MCP and Architecture DiagramsYour agent supplies the structure. A constraint solver decides where everything goes.
- Generate a Diagram from Your CodeEvery source below lists the exact files it reads and the constructs it maps to diagram nodes.
- AI Diagram GeneratorsThe agent emits a semantic graph and ELK computes the layout, so overlapping boxes are structurally impossible.
- Compare Lucid TrainEvery page says what the other tool is better at. If that section were empty the comparison would not be worth reading.
- Convert Code and DiagramsEvery conversion page states what survives the round trip and what does not.
- Use CasesEach page includes who it is not for, which is usually the more useful half.
- Beyond DiagramsDiagrams are the front door. These are the rooms behind it.