One or more of the following:
Abnormal tooth wear
Grinding sounds
Discomfort of the jaw musclesWith the following criteria supporting the diagnosis:

C. polysomnography shows both:
Activity of jaw muscles during sleep
No associated epileptic activity
D. No other medical or mental disorders (e.g., sleep-related epilepsy, which may cause abnormal movement during sleep). E. The presence of other sleep disorders (e.g., obstructive sleep apnea syndrome). Definition examples
Bruxism is derived from the Greek word βρύκειν (brykein) "to bite, or to gnash, grind the teeth". People with bruxism are called bruxists or bruxers and the verb itself is "to brux". There is no widely accepted definition of bruxism. Examples of definitions include:

"Bruxism is a repetitive jaw-muscle activity characterized by clenching or grinding of the teeth and/or by bracing or thrusting of the mandible. Bruxism has two distinct circadian manifestations: it can occur during sleep (indicated as sleep bruxism) or during wakefulness (indicated as awake bruxism). "All forms of bruxism entail forceful contact between the biting surfaces of the upper and lower teeth. In grinding and tapping this contact involves movement of the mandible and unpleasant sounds which can often awaken sleeping partners and even people asleep in adjacent rooms. Clenching (or clamping), on the other hand, involves inaudible, sustained, forceful tooth contact unaccompanied by mandibular movements. "A movement disorder of the masticatory system characterized by teeth-grinding and clenching during sleep as well as wakefulness." "Non-functional contact of the mandibular and maxillary teeth resulting in clenching or tooth grinding due to repetitive, unconscious contraction of the masseter and temporalis muscles."
Although the evidence of biofeedback has not been tested for awake bruxism, there is recent evidence for the efficacy of biofeedback in the management of nocturnal bruxism in small control groups. Electromyographic monitoring devices of the associated muscle groups tied with automatic alerting during periods of clenching and grinding have been prescribed for awake bruxism. Dental appliances with capsules that break and release a taste stimulus when enough force is applied have also been described in sleep bruxism, which would wake the person from sleep in an attempt to prevent bruxism episodes. "Large scale, double-blind, experiment confirming the effectiveness of this approach have yet to be carried out." Occlusal adjustment/reorganization
As an alternative to simply reactively repairing the damage to teeth and conforming to the existing occlusal scheme, occasionally some dentists will attempt to reorganize the occlusion in the belief that this may redistribute the forces and reduce the amount of damage inflicted on the dentition. Sometimes termed "occlusal rehabilitation" or "occlusal equilibration", this can be a complex procedure, and there is much disagreement between proponents of these techniques on most of the aspects involved, including the indications and the goals. It may involve orthodontics, restorative dentistry or even orthognathic surgery. Some have criticized these occlusal reorganizations as having no evidence base, and irreversibly damaging the dentition on top of the damage already caused by bruxism. History
Two thousand years ago, Shuowen Jiezi by Xu Shen documented the definition of Chinese character "齘" (bruxism) as "the clenching of teeth" (齒相切也).