The codon of a single complete protein molecule is called monocistronic mRNA.Sometimes, it contains several adjacent DNA cistrons and is knows as polycistronic mRNA.
The structural gene in a transcription unit could be said as monocistronic or polycistronic. In eukaryotes, the monocistronic structural genes have interrupted coding sequences because the genes in eukaryotes are split. The coding sequences or expressed sequences are defined as exons and introns.