A gene, a part of a gene, a group of genes, or fragments of many genes, on a molecule of DNA, a fragment of DNA, a group of DNA molecules, or fragments of many DNA molecules. Could refer to anything from a small fragment of DNA to the entire genome of an organism.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.