pre-noun adjectival
a certain; some
Pre-noun adjectival (rentaishi) used to refer to an unspecified but particular person, thing, or place. Always modifies a noun directly.
ある日、突然彼から電話がかかってきた。
One day, I suddenly got a call from him.
ある意味では、彼の言うことも正しい。
In a certain sense, what he says is also correct.
Conjunction meaning 'or' or 'perhaps'; derived from the same root but used differently.
Derived from the classical Japanese verb あり (ari, 'to be'), used attributively. The kanji 或 is a Chinese character borrowed for its meaning of 'a certain'.