diff --git a/src/runtime/define.ts b/src/runtime/define.ts
index d1a6cbe..a49f170 100644
--- a/src/runtime/define.ts
+++ b/src/runtime/define.ts
@@ -1,7 +1,6 @@
-import type {Variable, VariableDefinition} from "@observablehq/runtime";
+import type {Module, Variable, VariableDefinition} from "@observablehq/runtime";
 import type {DisplayState} from "./display.js";
 import {clear, display, observe} from "./display.js";
-import {main} from "./index.js";
 import {input} from "./stdlib/generators/index.js";
 import {Mutator} from "./stdlib/mutable.js";
 
@@ -31,7 +30,7 @@ export type Definition = {
   assets?: Map<string, string>;
 };
 
-export function define(state: DefineState, definition: Definition, observer = observe): void {
+export function define(main: Module, state: DefineState, definition: Definition, observer = observe): void {
   const {id, body, inputs = [], outputs = [], output, autodisplay, autoview, automutable} = definition;
   const variables = state.variables;
   const v = main.variable(observer(state, definition), {shadow: {}});
diff --git a/src/runtime/index.ts b/src/runtime/index.ts
index d1594ac..d3d77a4 100644
--- a/src/runtime/index.ts
+++ b/src/runtime/index.ts
@@ -1,20 +1,40 @@
 import type {Module} from "@observablehq/runtime";
 import {Runtime} from "@observablehq/runtime";
+import type {DefineState, Definition} from "./define.js";
+import {define as _define} from "./define.js";
+import type {observe} from "./display.js";
 import {fileAttachments} from "./stdlib/fileAttachment.js";
 import {library} from "./stdlib/index.js";
 
-export * from "./define.js";
+export type {DefineState, Definition} from "./define.js";
 export * from "./display.js";
 export * from "./inspect.js";
 export * from "./stdlib/index.js";
-
 export type * from "./stdlib/databaseClient.js";
-export type * from "./stdlib/fileAttachment.js";
 export {DatabaseClient} from "./stdlib/databaseClient.js";
+export type * from "./stdlib/fileAttachment.js";
 export {FileAttachment, registerFile} from "./stdlib/fileAttachment.js";
 
-export const runtime = Object.assign(new Runtime({...library, __ojs_runtime: () => runtime}), {fileAttachments});
-export const main = (runtime as typeof runtime & {main: Module}).main = runtime.module();
+export class NotebookRuntime {
+  readonly runtime: Runtime & {fileAttachments: typeof fileAttachments};
+  readonly main: Module;
+
+  constructor(builtins = library) {
+    const runtime = new Runtime({...builtins, __ojs_runtime: () => runtime});
+    this.runtime = Object.assign(runtime, {fileAttachments});
+    this.main = runtime.module();
+  }
+
+  define(state: DefineState, definition: Definition, observer?: typeof observe): void {
+    _define(this.main, state, definition, observer);
+  }
+}
+
+const defaultNotebook = new NotebookRuntime();
+
+export const runtime = defaultNotebook.runtime;
+export const main = defaultNotebook.main;
+export const define = defaultNotebook.define.bind(defaultNotebook);
 
 main.constructor.prototype.defines = function (this: Module, name: string): boolean {
   return (