This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (March 2016) (Learn how and when to remove this template message)
A thoracic vertebra. The facet joint is the joint between the inferior articular process (labeled at bottom) and the superior articular process (labeled at top) of the subsequent vertebra.
The facet joints, (or zygapophysial joints, zygapophyseal, apophyseal, or Z-joints) are a set of synovial, plane joints between the articular processes of two adjacent vertebrae. There are two facet joints in each spinal motion segment and each facet joint is innervated by the recurrent meningeal nerves.
The biomechanical function of each pair of facet joints is to guide and limit movement of the spinal motion segment. In the lumbar spine, for example, the facet joints function to protect the motion segment from anterior shear forces, excessive rotation and flexion. Facet joints appear to have little influence on the range of side bending (lateral flexion). These functions can be disrupted by degeneration, dislocation, fracture, injury, instability from trauma, osteoarthritis, and surgery. In the thoracic spine the facet joints function to restrain the amount of flexion and anterior translation of the corresponding vertebral segment and function to facilitate rotation. Cavitation of the synovial fluid within the facet joints is responsible for the popping sound (crepitus) associated with manual spinal manipulation, commonly referred to as "cracking the back."
The facet joints, both superior and inferior, are aligned in a way to allow flexion and extension, and to limit rotation. This is especially true in the lumbar spine.
Facet joint arthritis
In large part due to the mechanical nature of their function, all joints undergo degenerative changes with the wear and tear of age. This is particularly true for joints in the spine, and the facet joint in particular. This is commonly known as facet joint arthritis or facet arthropathy.  As with any arthritis, the joint can become enlarged due to the degenerative process. Even small changes to the facet joint can narrow the intervertebral foramen, possibly impinging on the spinal nerve roots within. 
Facet joint arthritis often manifests as a dull ache across the back. However like many deep organs of the body it can be experienced by the patient in a variety of referral pain patterns. The location of facet joints, deep in the back and covered with large tracts of paraspinal muscles, further complicate the diagnostic approach. Typically facet joint arthritis is diagnosed with specialized physical examination by specialist physicians. Advanced imaging such as MRI or CT may be necessary to confirm the diagnosis and to rule out other conditions.
Conservative treatment of facet joint arthritis involves physical therapy or osteopathic medicine, with muscle strengthening, correction of posture, and biomechanics being the key. More advanced cases can involve severe inflammatory responses in the Z-joint, not unlike a swollen arthritic knee. Corticosteroid injections may provide temporary pain relief anywhere from days to several months. With repeated injections, sometimes the patient may experience a more permanent improvement in their symptoms. Steroid injections are typically performed under image guidance to ensure accuracy given the complex shape and deep location of the facet. Radiofrequency ablation or lesioning, also known as rhizolysis, can be used to give longer lasting relief by destroying the nerves that supply the facet joint (medial branch nerves). Finally, surgery in the form of a facetectomy can be performed in certain cases, particularly when the nerve root is affected.
Ancient Greek: zygon ("yoke") + apo ("out/from") + phyein ("grow")