The state of a cell's or organism's genome where one or more extra copies of the genome exist. For example, a genome that is normally 2n (contains two copies of its genome, one from 'mom' and one from 'dad') undergoes a whole genome duplication event to give rise to cells that contain 4n. This is also known as a tetraploid (four genomes). Many plant genomes are polyploid, and most (all sequenced to date) plant genomes contain ancient polyploid events. Please see this analysis of maize and sorghum for an example of a plant tetraploidy.