1. a condition, resulting from persistent use of a drug, characterized by a markedly diminished effect with regular use of the same dose of the drug or by a need to increase the dose markedly over time to achieve the same desired effect. Tolerance is one of the two prime indications of physical dependence on a drug, the other being a characteristic withdrawal syndrome (see substance withdrawal). Development of drug tolerance involves several mechanisms, including pharmacological ones (i.e., metabolic tolerance and pharmacodynamic tolerance) and a behavioral one (i.e., a behavioral conditioning process). Also called drug tolerance. See substance dependence.

2. acceptance of others whose actions, beliefs, physical capabilities, religion, customs, ethnicity, nationality, and so on differ from one’s own.

3. a fair and objective attitude toward points of view different from one’s own.

4. permissible or allowable deviation from a specified value or standard. —tolerant adj.