Tolerance is generally the increased amounts of a drug needed to achieve intoxication, or a diminished drug effect with its continued use.

It is the extent to which a person requires larger and larger amounts of a substance in order to achieve its desired effects, or the extent to which the individual feels less of its effects after using the same amount of the substance. It is the need for increasing levels of a drug in order to produce a constant level of effect. Moreover, it also refers to the amount beyond which pain becomes unbearable and intolerable; a progressive decrease in responsiveness to a drug with continued use.

Other /More definition:
tolerance refers to the need for increased amounts of a substance to achieve the desired effect, and a diminished effect with continued use of the same amount.