Difference between a cistron and a exon

Cistron : A segment of DNA coding for a polypeptide; the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes).
Exon : The coding sequences or expressed sequences are defined as exon. Exons are said to be those sequences that appear in mature or processed RNA.