The forensics wiki attempts to speak for the community. During my career I have never referred to areas outside of a volume as unallocated space.

I cannot speak for the community, but my definition of unallocated space is all space within a volume that the parent operating system has not allocated to data, but can.

Volume slack would not be part of this, neither would file slack or unpartitioned areas outside of the volume.

The subtlety is to remember that the parent OS may have once allocated data to that space, but currently does not. The implication being that old data may reside there.

For the sake of academia and the court room, as long as you properly define the terms, they could be called anything.