An Elvish term of address simply meaning 'my child'. Its only recorded use is between Meneldur and his son Aldarion (as in the quote above), but there is reason to believe that it was more widely used, at least among noble Men. The complementary term 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.