From the Queensland Institute of Medical Research (QIMR) Glossary of Terms:

"Reporter gene - a gene that codes for a product that can readily be measured, such as a fluorescing protein.  The fluorescence generated by the protein can indicate, for example, whether a DNA construct has been successfully introduced into a cell. GFP (green fluorescing protein) is one example used in flow cytometry and fluorescence microscopy."

From the Baylor College of Medicine Human Genome Sequencing Center Glossary:

 

Reporter gene is a coding unit whose product is easily assayed (such as chloramphenicol transacetylase); it may be connected to any promoter of interest so that expression of the gene can be used to assay promoter function."

 

From the South African Agency for Science and Technology Advancement (SAASTA) Public Understanding of Biotechnology  Resources Glossary:

 

"Reporter Gene: A gene that is inserted into DNA so that cell will "report" (to researchers) if a linked gene is functioning properly in the transgenic organism."

 

From the Australian Prescriber:

 

"Reporter gene - A gene whose product can be used as a genetic `label'. For example, a gene for neomycin resistance incorporated into a plasmid before transfection allows the subsequent detection of successfully transfected cells."

 

From Wikipedia:  The Free Encyclopedia:

"In molecular biology, a reporter gene (often simply reporter) is a gene that researchers attach to another gene of interest in cell culture, animals or plants.  Certain genes are chosen as reporters because the characteristics they confer on organisms expressing them are easily identified and measured, or because they are selectable markers.  Reporter genes are generally used to determine whether the gene of interest has been taken up by or expressed in the cell or organism population."