It is, if you decide it is. But it's not a string in the C language. For it to be a string it HAS to be terminated with the null character. As jremington states, it's required for the string functions in the standard library to function properly.
What a string is in the C language is really just convention/tradition, other languages implements string functionality in different ways. You can even develop your own string implementation in C if you really want to.