Who devised the term elements?
One answer is that the ancient Greeks, specifically Plato, devised the term "element".  Quoting from wikipedia article on Classical elements:


  Plato seems to have been the first to use the term "element (στοιχεῖον, stoicheion)" in reference to air, fire, earth, and water. The ancient Greek word for element, stoicheion (from stoicheo, "to line up") meant "smallest division (of a sun-dial), a syllable", as the composing unit of an alphabet it could denote a letter and the smallest unit from which a word is formed. A similar alphabetic metaphor may be the origin of the equivalent Latin word elementum (from which the English word comes), possibly based on the names of the letters 'l', 'm', and 'n', though the validity of this idea is debated.


If you had a more modern, chemical definition of "element" in mind for your question, then another possible answer is Robert Boyle.

Quoting from wikipedia again, this time from its article on chemical elements:


  In 1661, Robert Boyle proposed his theory of corpuscularism which favoured the analysis of matter as constituted by irreducible units of matter (atoms) and, choosing to side with neither Aristotle's view of the four elements nor Paracelsus' view of three fundamental elements, left open the question of the number of elements. The first modern list of chemical elements was given in Antoine Lavoisier's 1789 Elements of Chemistry, which contained thirty-three elements, including light and caloric.


I can't resist adding a reference to my favorite English-language modern chemistry text that omits the word "element" entirely.  


  For most of its being, mankind did not know what things are made
  of, but could only guess. With the growth of worldken, we began
  to learn, and today we have a beholding of stuff and work that
  watching bears out, both in the workstead and in daily life.
  
  The underlying kinds of stuff are the firststuffs, which link
  together in sundry ways to give rise to the rest. Formerly we
  knew of ninety-two firststuffs, from waterstuff, the lightest and
  barest, to ymirstuff, the heaviest. Now we have made more, such
  as aegirstuff and helstuff. [...]


If the Normans had never invaded England in 1066, we might not have "elements" at all, but "firststuffs" instead.  Either way, it's just a debate about the right translation for "stoicheion", a concept which Plato seems to have given us.