What is the definition or description of: tolerance?

When dealing with: Radiotherapy this is the dose that normal tissues tolerate, acutely or over time.

Drug dependence: When one takes a drug, such as heroin or alcohol, repeatedly, 'tolerance' develops because more of the drug is needed to give the same effect on the body/brain; e.g. Liver enzymes r stimulated to metabolize the drug faster and faster. There may be a strong genetic predisposition to addiction, some studies suggesting that 1 of 10 people will become dependent on alcohol or some drug after small uses.