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Onya is a Quenya word with two possible meanings:

  1. a term of address simply meaning 'my son'. Its only recorded use is between Meneldur and his son Aldarion, but there is reason to believe that it was more widely used, at least among noble Men. Its opposite was Atarinya, "my father", and we know that this was used by the Exiles of Númenor long after Meneldur's time, so it's reasonable to assume that the word onya survived in use at least into the Third Age, too.
  1. a word for Ent.

Tolkien wrote that the normal word for "child" is onna or onwe, and that onya (which can be used to address either a son or daughter) is formed with the 1st person possessive suffix, on-nya "my child", comparable to atya "my father" and amya "my mother.[1]

  1. J.R.R. Tolkien, "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", in Parma Eldalamberon XVII (edited by Christopher Gilson), p. 170