In the Debian project they refer to packages that no longer have mantainers as orphaned. I think it’s a good definition, and I’d extend it to free software packages that are no longer developed.

There are a lot of orphaned packages around, some actually deserve it but unfortunately there are also some that are promising or very good, and now they are almost dead. But, since we are talking about free software, every good developer is encouraged to pick one and try to push it a bit further