> ## Documentation Index
> Fetch the complete documentation index at: https://docs.eucaengine.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Glossary

> The core Euca terms, defined against the code — ECS, the world-engine verbs, rules, and the evaluation vocabulary.

Definitions grounded in the source. For where each lives, see the linked concept and system pages.

## ECS

* **Entity** — a generational handle `{ index, generation }`. Despawn bumps the generation, so a stale
  reference is *detected*, not silently reused. Carries no data itself.
* **Component** — plain data on an entity (`Transform`, `Velocity`, `Health`, `Team`, …); any
  `'static + Send + Sync + Clone` type. The [world is a flat table](/concepts/world-as-table) of these.
* **Archetype** — entities are stored columnar (SoA) grouped by their exact set of components, for cache-
  friendly iteration. See [Engine internals](/concepts/engine-internals).

## The world-engine verbs

* **observe** — read the full current state exactly, without stepping (`POST /observe` →
  `RichEntityData`). [→](/concepts/query-verbs)
* **peek** — the *exact* probability distribution over the next step's declared outcomes, read
  analytically (not sampled) from the generative model. The answer key.
* **step** — advance one tick deterministically, sampling from *exactly* the distribution `peek`
  reported (`POST /step`).
* **fork** — clone the world mid-episode and branch a counterfactual (the do-operator);
  `POST /fork`. [→](/concepts/forks)
* **`peek == step`** — the soundness invariant: a named test holds the two distributions byte-identical.
  A perfect predictor scores 0 nats.

## Data & state

* **WorldStateGraph** — the observable world as `{ tick, entity_count, entities }` in canonical JSON;
  the structured form behind `observe`. [→](/systems/datasets)
* **RichEntityData** — the flattened per-entity projection `observe` returns over HTTP.
* **state\_digest** — a 64-bit FNV-1a hash of the observable state over a canonical ordering. Equal
  digests ⇔ equal observable state — the reproducibility oracle.
* **JointDistribution** — the per-variable distributions `peek` returns (`per_variable: [(name, dist)]`).

## Rules

* **Rule** — `{ when, do }` as pure data: a condition, an optional declared distribution, and effect
  bundles. No closures. [→](/concepts/rules-as-data)
* **RuleSet** — the ordered collection of rules the engine evaluates each tick; rule firing order is
  deterministic.
* **GameAction** — an effect an action can apply (`Spawn`, `Damage`, `Heal`, `Teleport`, `EndGame`,
  `SetField`, `AddField`, …; `SetColor`/`ShowText` are render-gated stubs).

## Determinism & evaluation

* **determinism (within-platform)** — same seed + inputs ⇒ bit-identical trajectory, within one
  build/platform; *not* a claim of identical floating-point across CPU architectures. [→](/concepts/determinism)
* **OnlineWorld** — the live world + the four-verb loop + a single seeded RNG stream, in `euca-online`.
* **regret (nats)** — the prequential score: `Σ −ln P̂(realized) − Σ −ln P*(realized)`. ≥ 0 in
  expectation; a perfect predictor scores 0. [→](/evaluation/overview)

<Card title="Core concepts" icon="book" href="/concepts/world-as-table" horizontal>
  Start with the world as a table.
</Card>