[A]lthough these days "byte" is synonymous
with "8 bits", historically it
meant "the number of bits needed to store a single character".
It depends upon what you mean by "historically". Originally "byte"
was coined to refer to 8 bit addressable units on the IBM 7030 "Stretch"
computer. The term was perpetuated for the 360 family of computers. Only
later did people begin to attribute the meaning to non-addressable
6- or 9-bit units on 36- and 18-bit machines.
Viewed over history, the latter usage was transient and colloquial.
Doug