Its antonym is polycistronic.
It is often stated that all eukaryote mRNAs are monocistronic, but the chloroplasts of plants for instance contain polycistronic transcripts, e.g. the psbb/psbH/petB/petD region.
Monocistronic mRNA are in some cases (e.g. viruses) translated to form a polyprotein, which is processed by proteolytic cleavage to form the mature gene products.