This Motive has been given multiple titles over time.  It is interesting to note that Gustav Kobbé labels Motive 31 “Ecstasy,” even though he gives the same name to what we know as "Rapture (a)" as well.

Motive Labels
“Isolde’s Blissful Self-Abandon” – (Redlich, p. 27)
“Liebesverklärungs” or “Love’s Transfiguration” – (Windsperger, p. 44)
“Verklärtes Sehnen” or “Longing’s Transfiguration” – (Pfohl, p. 215)
“Ecstasy” – (Kobbé, p. 127)