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).
某国の大統領が来日した。
The president of a certain country visited Japan.
某有名企業に勤めている。
I work for a certain well-known company.
Archaic humble first-person pronoun used mainly by men. In modern Japanese, this usage is virtually obsolete and appears only in historical or stylized contexts.
See also: 某 (それがし)
某はその儀、承知仕った。
I am aware of that matter.
某 as a pronoun is a rare, highly literary archaic term for 'I' used mainly by men, while それがし is a more commonly seen archaic first-person pronoun in historical texts.
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.