I don't think I've ever seen a definition for that - it looks like it's just the response time from that point in the transaction though. In this case from the end user/browser pespective.
As to cause I can't say, but to make sure you're interpreting that correctly what elapsed time means is how many ms after the start of that user action purepath before that request was initiated so in this case it was 18 seconds after the start of the action before that /SelectOrderForm.aspx was initiated and it seems to have been pretty quick at 6.57 ms. What I would be interested in is it looks like it was waiting for that /NotesForm.aspx to complete which took 17 seconds. I'd drill into that server side purepath and see where the time is spent to begin troubleshooting.
Right click on that web request and select drill down -> purepaths. It will take you to the server side purepath where you can analyze the method tree and the other things like the hotspots pane to see where time is spent. This doc helps to explain all that is on there:
Note that you can also drill down to dashlets like method hotspots and response time hotspots to help analyze where time is spent. Just looking at the size of that agent group name in your flow screenshot it looks like a lot of time is spent in IIS.