A test fixture is something used to consistently test some item, device, or piece of software.
Test fixture refers to the fixed state used as a baseline for running tests in software testing. The purpose of a test fixture is to ensure that there is a well known and fixed environment in which tests are run so that results are repeatable. Some people call this the test context.
Examples of fixtures:
- Loading a database with a specific, known set of data
- Erasing a hard disk and installing a known clean operating system installation
- Copying a specific known set of files
- Preparation of input data and set-up/creation of fake or mock objects
Test fixture in xUnit 
In generic xUnit, a test fixture is all the things that must be in place in order to run a test and expect a particular outcome.
Frequently fixtures are created by handling setUp() and tearDown() events of the unit testing framework. In setUp() one would create the expected state for the test, and in tearDown() it would clean up what had been set up.
Four phases of a test:
- Set up -- Setting up the test fixture.
- Exercise -- Interact with the system under test.
- Verify -- Determine whether the expected outcome has been obtained.
- Tear down -- Tear down the test fixture to return to the original state.
Use of fixtures 
Some advantages of fixtures include separation of the test initialization (and destruction) from the testing, reusing a known state for more than one test, and special assumption by the testing framework that the fixture set up works.
Physical testing 
In physical testing, a fixture is a device or apparatus to hold or support the test specimen during the test. The influence of test fixtures on test results is important and is an ongoing subject of research.
Test fixture on universal testing machine for three point flex test
Some fixtures employ clamps, wedge grips and pincer grips.
Further types of construction are eccentric roller fixtures, thread grips and button head grips as well as rope grips.
Mechanical holding apparatus provide the clamping force via arms, wedges or eccentric wheel to the jaws. Additional there are pneumatic and hydraulic fixtures for tensile testing that do allow very fast clamping procedures and very high clamping forces
See also 
- Abadalah, MG; Gascoigne, HE (1989). The Influence of Test Fixture Design on the Shear Test for Fiber Composite Materials. ASTM STP.
- ASTM B829 Test for Determining the Formability of copper Strip
- ASTM D6641 Compressive Properties of Polymer Matrix Using a Combined Loading Compression Test Fixture