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I'm happy you asked! :) I think these terms are very vague, and I haven't found a good way to be more clear. For me a "real repl" includes:

1. Decent support from the language to do REPLy things

E.g. being able to change a function during runtime. Imagine an animation which is defined by perhaps: `(defn anim [time v] (* time v))`. Now you want to try something else, without having to recreate all state, so you just do `(defn anim [time v] (* time v v))` and hit Ctrl+L ("load file"). Oh, it seems I need to reset the animation, so I add some code to reset just the part of the state needed. Now I'm able to hit Ctrl+L and it'll run the new animation each time. The ability to do this ad-hoc anywhere in my code is so flow-inducing it's crazy. And I do this for games, web servers and browser GUIs. It's surprisingly general!

2. Editor support

The Ctrl+L I mentioned above is something I do in my editor to send code from my editor to the repl server. I haven't tried Ipython, but I'm guessing it might work similarly to node, where the client and server are kind of stuck together. With a "real repl", you generally use your editor as the client. Here's a video I've made that shows how this can look: https://youtu.be/Enumt6qxAgc?t=667

I know it's often possible to disconnect these (I've tried connecting my editor to the chrome debugger -- that's pretty cool!), but it doesn't seem to be in common use, which leads to...

3. Libraries supporting being run from REPLs

One not-so-uncommon problem is that e.g. a web server or game loop might block the REPL. This one is probably the hardest one to fix for existing languages and their libraries. There are often ways around this, like running on multiple threads, but that can lead to new problems. Even in clojure, I've had problems due to java libraries blocking (e.g. game lib LWJGL). It doesn't help that on macos all GUI related calls must be done on the main thread (so running LWJGL on one thread and repl on other breaks things). So far with janet I've managed to solve these issues easily thanks to the event loop (which iiuc works similar to js event loop).

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I think the main thing to realize is that this way of developing leads to new possibilities of exploring software development. Suddenly you're able to experiment quicker, in a way that sometimes leads to typing speed being a bottleneck! :D It feels really cool, and I want to share this feeling of flow with as many as possible.