An ISV is...
An Independent Software Vendor,
also known as software authors.
They are software developers who sell software that they (usually) write themselves.
Some use the term "MicroISV", which means very small companies...
sometimes only the software developer alone, sometimes him/her and few partners,
or maybe him and his reflection in the mirror.
Often, ISVs are self-funded start-ups. They don't have big investors,
instead they have an idea, a computer, and a will to work for themselves.
In times past, ISVs were called Shareware Authors. (That was before "shareware" became a dirty word.)
In 10 or 15 years, there will probably be a new way to describe us.
But for now, the generally accepted term is "ISV".