"Molecular farming" is the use of agriculturally important plants to produce biomolecules instead of food and fiber. Transgenic plants which have been genetically engineered to make scientifically, medically, or industrially useful biomolecules (such as medicinal proteins or plastics) are grown as crops and harvested for the biomolecules.
The information in this writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/