Penstemon hallii
Scrophulariaceae (Snapdragon Family)

Almost all botanical sources  indicate that the word "Penstemon" is derived from the Greek "pente" ("five") and "stemon" ("thread").  Penstemons are so named because one of their five stamens, i.e., one of their thread-like structures, is sterile and visually distinctly different from the others. 

An alternate, but ultimately similar etymology is given by Intermountain Flora: "paene" is Latin for "nearly" and thus the name means "nearly a stamen", i.e., one of the five thread-like structures is almost a true stamen.  The botanical word for such a structure is "staminode", "a modified stamen which does not produce pollen".  (Intermountain Flora definition.)