Test Bench's central abstraction and its most novel capability. A fixture is just a plain Ruby class that includes the `TestBench::Fixture` module and typically exposes a `call` method — there is no special matcher API. Because the Test Bench DSL operates in isolation from the framework, a fixture inherits full access to context, test, assert, refute, indentation, and output methods, and can itself be tested (you can prove it passes when it should and fails when it should). Fixtures compose: higher-level fixtures invoke lower-level ones (e.g. a schema-equality fixture reused by a message-handler fixture comparing events). The name comes from Kent Beck's 1989 Smalltalk xUnit writeup — a behavioral 'device for testing something' — not from Rails/Active Record YAML fixtures. Ladd uses the light-fixture metaphor: a fixture powers and holds the thing being tested. Fixtures let teams build exactly the verification tool they need (e.g. nokogiri+CSS-selector XML assertions, JSON attribute inspection, entity comparisons) rather than relying on general-purpose matcher DSLs.