Leptin is a protein hormone which has been linked to obesity. It is produced in adipocyte (fat cell) tissue by the obese (ob) gene and probably acts as a signal to the brain on the amount of fat currently in the body.
In obese people, the leptin receptor is thought to be faulty so that much higher levels of leptin need to be secreted before the brain turns off the hunger signal.
From the science dictionary at http://biotech.icmb.utexas.edu/