A particular type of growth curve seen in cultured microorganisms in which they have two exponential growth stages separated by a plateau phase. This double-hump curve is produced when the microbes are cultured using two carbon sources, one of which must be used up before the second can be used.

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