Star Trek: Defiant was an ongoing audio drama fan show produced by Pendant Productions.
The ship, and her crew, were later featured in an alternate timeline in the Darker Projects audio drama Star Trek: The Section 31 Files. This is, however, not considered canon to official Defiant storyline.
The show's history dates back to 1995, when Jeffrey Bridges and a groups of fellow Star Trek fans met online in an endeavor to start an online mailing list of Star Trek fan fiction. The crew consisted of the members on the list, and were stationed on the newly commissioned federation starship Defiant (NX-74205). This is a different starship from the one featured in Star Trek: Deep Space Nine, as it is the federation ship USS Sao Paulo recommissioned after the apparent destruction of the original USS Defiant.
Around 2003, a group on the mailing list discussed the possibility of remaking their fan stories into an audio drama. As a result, a new audio production company named Pendant Productions emerged. Since then, the old mailing list has been decommissioned, and the series has lived on as an audio drama.
On May 29, 2012, Pendant ceased production on all fan shows and voluntarily removed them from their website. 
- Star Trek: Defiant Official Wiki Page
- Pendant Productions Defiant Page