expression, noun
someone; a certain person
Refers to an unspecified or unnamed person, often used when the speaker does not know or does not wish to specify who. Common in both spoken and written Japanese.
ある人が私にそう言った。
Someone told me that.
ある人にとっては簡単なことでも、他の人には難しい。
What is easy for one person may be difficult for another.
誰か is the most common word for 'someone' and is used in questions and statements. ある人 is more literary or formal, and often implies a specific but unnamed person.
Combination of the prenominal adnominal ある (a certain) and 人 (person).