Definition (general):
Human biological monitoring is a procedure to determine if and to what extent chemical substances from the environment are absorbed in the body.
Definition (more precise):
Human biological monitoring is the is the standardized and repeated systematic collection, pretreatment, storage, and analysis of body tissues to assess the internal dose of a xenobiotic substance by analysis of the parent substance or a product of biotransformation.
For more background on human biological monitoring please refer to:
http://en.wikipedia.org/wiki/Biomonitoring
http://en.wikipedia.org/wiki/Biological_monitoring_working_party