In *theory* a "Kush" strain is an plant with genetics derived entirely, or at least in significant part to cannabis strains from the subcontinent, around the Kush mountain range in what is now Afghanistan/Pakistan. These are the short, bushy, broad leaved, resinous, narcotic-like pure Indicas, traditionally grown in dry climates and renowned for high-quality hashish production. These "true" Kush plants tend to have relatively fast flowering times of only about 7 weeks, give or take a bit, and yeah, those genetics are available if you want to get them. Commercially available examples of these "true Kush" plants would include Afghani #1, Maple Leaf Indica, Hindu Kush, Petrolia Headstash, and others.
In *practice*, since there is no regulation nor standardization in the cannabis seed industry, the term "Kush" has become somewhat of a marketing term, without necessarily being attached to a specific genetic lineage.
Most of the so-called "Kushes" are simply potent indica/sativa hybrid plants, and so while most of them do have true "Kush" genes in there, the same could be said of pretty much any indica hybrid, including many that aren't called "Kush". Some of the labelled "Kush" strains are excellent, and some aren't, and a lot of the ones with different names really aren't all that different from one another.
I'd say forget the word "Kush". If you're looking for a fast-flowering strain, there are plenty of good ones out there, and they may (or may not) actually be labelled "Kush".