"Transwarp" in TNG era seems to mean two things. It can be used as a generic term for any mode of propulsion that achieves speeds not practical under the existing warp scale, but still beneath the "infinite velocity" Warp 10. Slipstream would therefore be a type of transwarp, expressible on the conventional scale (probably as Warp 9.9999999999 or something, given its an exponentional scale) but not achievable by current ships.
Transwarp can also specifically refer to the Borg (but presumably not Borg-constructed) network of ancient, pre-established conduits spread throughout the Milky Way and somehow held together by a number of those "hub" things, one of which Voyager destroyed in their series finale. It's powerful, but limited in that you can only use it to go where conduits have already been built... unless you have the knowledge to establish new ones. But that likely has to be done at sub-Transwarp speeds meaning it could take centuries to lay down conduits to new locations. That's probably what's mostly kept the Borg confined to the Milky Way, whereas a technology like Slipstream could've enabled them to go anywhere they please at similar velocities.
Edited by TM2-Megatron, 20 January 2021 - 02:53 PM.