diff --git a/src/runtime-definition.ts b/src/runtime-definition.ts
new file mode 100644
index 0000000..09126f1
--- /dev/null
+++ b/src/runtime-definition.ts
@@ -0,0 +1,28 @@
+import { availableParallelism } from 'node:os';
+import { define } from './define.ts';
+import type { Balancer } from './runner.ts';
+
+/**
+ * Configuration for a global runtime, defined in a side-effect-free module
+ * via `define()` so every thread can resolve the identical config by module
+ * identity. Workers never receive the config by serialization — they import
+ * the defining module (follow-up plan); on the main thread it configures the
+ * lazily-booted global pool.
+ */
+export interface RuntimeDefinition {
+  /** Number of worker threads. Defaults to `os.availableParallelism()`. */
+  size?: number;
+  /** Load balancing strategy. Defaults to least-busy. */
+  balance?: Balancer<any>;
+  /** Max ms to wait for in-flight tasks during `runtime.shutdown()`. */
+  shutdownTimeout?: number;
+}
+
+/**
+ * Moroutine's built-in runtime definition — used when no custom definition is
+ * registered. It is "just another runtime": the only default-vs-custom
+ * difference anywhere is which definition boots.
+ */
+export const defaultRuntime: RuntimeDefinition = define(import.meta, {
+  size: availableParallelism(),
+});
diff --git a/test/runtime-definition.test.ts b/test/runtime-definition.test.ts
new file mode 100644
index 0000000..fd47ca0
--- /dev/null
+++ b/test/runtime-definition.test.ts
@@ -0,0 +1,18 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { availableParallelism } from 'node:os';
+import { defaultRuntime } from '../src/runtime-definition.ts';
+import { getDefineId } from '../src/define.ts';
+
+describe('defaultRuntime definition', () => {
+  it('is a define()-branded value', () => {
+    const id = getDefineId(defaultRuntime);
+    assert.ok(id !== undefined);
+    assert.match(id!, /runtime-definition\.ts#\d+$/);
+  });
+
+  it('carries default config', () => {
+    assert.equal(defaultRuntime.size, availableParallelism());
+    assert.equal(defaultRuntime.balance, undefined); // undefined → pool default (leastBusy)
+  });
+});