Biological half-life (represented as t 1/2) is the time required for one-half of the total amount of a particular substance in a biological system to be consumed, metabolized, or otherwise broken down by biological processes when the rate of removal is approximately exponential.
In human medicine and pharmacokinetics, this term refers to the amount of time it takes the body to eliminate half of the amount of a chemical after it has been absorbed by the body.
Toxic chemicals with a long biological half-life (such as some pesticides) will tend to accumulate in the body and are, therefore, more likely to be harmful. A substance with a short biological half-life may still accumulate if a portion of it it becomes tightly bound to bone or other tissues, even if most of it is quickly cleared from the body.
From the science dictionary at http://biotech.icmb.utexas.edu/