williamseal - No one seems to have answered your question about the title. You'll find an explanation if you look up in Chambers the definition of the word which describes the 5 wordplay-only answers and the shape of the curve.
Back to Aedites's actual crossword - can someone please explain the parsing of 3dn - I have completed the crossword, highlighted the name and drawn the curve - but I cant' resolve 3dn's last letter - it can only be one of two letters but .......?