Ecclesiastes is the translation in the Greek Bible called the Septuagint of a Hebrew word, Koheleth, whose meaning it is hopeless to fix with accuracy. All that we can say is that the word has something to do with a congregation or and assembly, hence the suggested translations

a leader of an assembly,
or
the assembly itself,

a debater,

a preacher,

a great orator.
Of these translations,
an assembly
might seem to fit in best with the nature of the contents of a book which is an assemblage of different conceptions of life and conduct. The final literary editor, whoever he was, of this book no doubt intended it to appear as a whole, and, perhaps, for this purpose, employed considerable editorial craftsmanship; but his labours have created endless difficulties for the later interpreters, who have been driven to desperate straits to reconcile the conflicting and contradictory views which the book, as a whole presents…. From the Introduction