The cis-trans test (also known as the complementation test) is a lab test which is used to determine whether two mutations of different genes which affect the same phenotype are on the same functional unit (indicating a cis configuration of the mutated genes) or on different functional units (indicating a trans configuration of the mutated genes). A functional unit can be a chromosome.
The test is done by mating an individual that has one of the mutations to an individual that has the other one, and observing whether their offspring have the mutant phenotype. If the offspring do not have the mutant phenotype, then the genes are known to be trans, because the offspring have normal copies of each mutant gene on the different functional units which are able to genetically complement each other. If the offspring do have the mutant phenotype, then the genes are known to be cis, because the offspring will always inherit at least one of the mutant genes on the one functional unit, resulting in the mutant phenotype.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.