Motive 20 is usually referred to as the “Impatience” motive.  Windsperger refers to this motive and the Torch by the single name “Liebesverlangen” or “Love’s Demand” (Windsperger, p. 42).  He labels them as such because the two motives occur simultaneously in treble and bass voices.  Additional labels for Isolde's Impatience are noted below.

Motive Labels
“Isolde’s Impatience” – (Fischer, p. 12) and (Redlich, p. 24)
“Motiv ungeduldiger Erwartung” or “Motive of Impatient Expectation” – (Windsperger, p. 42)
“Impatience” – (Kobbé, p. 122) and (Lavignac, p. 297)
“Lauschen, Ungeduld, Verlangen” or “Listening, Impatience, Demanding” – (Pfohl, p. 221)
Bass part of “Liebesverlangen” or “Love’s Demand” – (Windsperger, p. 42)

Isolde's Impatience – Order of occurrence by Act and Scene
Act II: Prelude, Scenes 1 & 3