Used as a prefix or noun to refer to an unspecified person, place, or thing, often in formal or written contexts. Common in phrases like 某国 (a certain country) or 某氏 (a certain person).
The president of a certain country visited Japan.
I work for a certain well-known company.
I am aware of that matter.
某 is a formal prefix/noun for 'a certain' used in written or official contexts, while ある is the everyday prenominal word used before nouns like ある日 'one day'.
a certain place; somewhere
a certain person; Mr. So-and-so; an unnamed individual
a certain country; some country
a certain company; an unnamed company
so-and-so; a certain person
Originally a Chinese character meaning 'certain' or 'some', borrowed into Japanese with the same function. The pronoun usage developed from the idea of 'a certain person' referring to oneself humbly.