It includes the phonetic units counted in haiku, tanka, and other such poetic forms.
The concatenation of the two words on and ji into onji (音字) was used by Meiji era grammarians to mean "phonic character" and was translated into English by Nishi Amane in 1870 as "letter".
Since then, the term "onji" has become obsolete in Japan, and only survives in foreign-language discussion of Japanese poetry.
It was taken up after a 1978 letter to Frogpond: Journal of the Haiku Society of America decrying the then-current use of the word "jion", which itself appears to have arisen in error.
The disjunction between syllables and on becomes clearer when counting sounds in English-language versions of Japanese poetic forms, such as haiku in English.