I have always used the word otona to signify adult. Recently, I have seen the word shakaijin. Is there a significant difference between the two words？ It seems like shakaijin is a member of society. Can I use both words interchangeably?
- 大人 is "adult", someone who is mature enough. It typically refers to anyone over 18 or 20.
- 社会人 is someone who has started to work regularly. It's mainly used in business contexts. A 22-year old university student is usually not called a 社会人 yet. So 社会人 is somewhat closer to a (responsible) businessperson, although 社会人 also includes farmers, novelists and such.
See also: What's the difference between 成人 and 大人?