Meaning and functionality of heḇel in Qoheleth

The Hebrew noun הבל which opens and closes the book of Qoheleth is a key term; yet its exact meaning has long been debated. Most studies have attempted to understand the term by its etymology, its function, its metaphorical usage, or by its context. Three main sections shape this book.
If the first section lays the foundation for the investigation on the meaning of heḇel, the second part offers an examination of Qoh 1:2-11; 12:1-8, as they both describe the fixity of human nature and the fleetingness of human experience in a fleeting world. Thus, using texts as his basis, the author of this book substantiates, in the last section, his view that הבל functions in Qoheleth predominantly as a metaphor for brevity of life, the non-lasting aspect or effects of things. For Qoheleth, life is not vanity, meaningless, but is fleeting.