Wiktionary
gene cassette
n. A DNA sequence which can exist by itself in circular form, or can be integrated into an integron.
Wikipedia
Gene cassette
A gene cassette is a type of mobile genetic element that contains a gene and a recombination site. They may exist incorporated into an integron or freely as circular DNA. Gene cassettes often carry antibiotic resistance genes. An example would be the kanMX cassette which confers kanamycin (an antibiotic) resistance upon bacteria.