What is the definition or description of: half-life?

Time drug is active.: The half-life of a drug is a measurement of time from the drug entering the blood stream until the body (usually the liver or kidneys) metabolizes 1/2 of the drug into an inactive form. So, if the alcohol level in the body is 1%, the period of time for that level to become 1/2 % is the half-life of that amount of alcohol in that person, at a particular time.