Definition
An ISV, or independent software vendor, is a company that builds and sells software that runs on or connects to another company's platform. In partner programs, ISVs are usually technology partners. They integrate with a vendor's product so both products work better for shared customers.
Large platform companies often run separate ISV programs with technical resources, certification and marketplace access. Smaller vendors may treat ISVs as part of a technology alliance program.
The value of an ISV partnership usually depends on the depth of the integration and whether the two sales teams actually work deals together.
In practice
A vulnerability scanning company builds an integration that sends findings into a ticketing platform, then joins that platform's ISV program to get listed in its app directory.