The word "heliotropism" has roots in ancient Greek, combining "helios," (ἥλιος) meaning "sun," and "tropism" (τροπή), meaning "a turn" or "a turning."  Therefore, heliotropism literally translates to "sun-turn" or "turning towards the sun". This accurately describes the phenomenon where plant parts, particularly leaves and flowers, orient themselves in the direction of the sun's movement across the sky.  The term itself origninated in French as "héliotropisme" and was most likely introduced by the Swiss naturalist Jean Senebier around 1800, before being adopted by the English.  

According to the Oxford English Dictionary (OED), the noun "heliotropism" was first used in English in 1854, in the writings of Robert Mayne. 

Examples of "heliotropism" used in a sentence: 
"Heliotropism can be seen in sunflower fields as the plants track the sun's movement across the sky." |
"The researchers have not yet identified the genes involved in heliotropism." 
"It's heliotropism, and sunflowers are not the only plants that track the sun."
"But light sensitivity—or heliotropism—plays a role too." 
"All legume plants do this, so be sure to watch your peas and beans for signs of heliotropism." 
"His very geographical situation was sufficient to turn the mind towards him, but the particular reason for that heliotropism  on the part of his feminine neighbors was that he was an easy man for a woman to ask." 

Just the facts:

HELIOTROPISM Definition & Meaning - Merriam-Webster