一理ある basically means “(someone’s argument) is true in a way”. It is often used when you show some recognition of someone’s statement while providing an opinion to counter it, as in “Xは一理あるが、しかしY” meaning “X is true in a way, but Y”. It is also used when you are more or less convinced by someone’s statements and accept them.
Nowadays, young people use 一理ある very casually on the Internet when they casually agree with someone. In this case, it means “That’s true”, “You’re right” or “agreed”.
一理ある consists of the two parts “一理” and “ある”. The former literally means “one rationale”, and the latter is a common word that means “(a thing) exists”. Therefore, 一理ある indicates that there exists at least one rationale or truth behind someone’s statement.
Some people say “Today there is no need to go to university because we can study by ourselves whatever we want to on the Internet.” This statement 一理ある (is true in a way), but we should not forget that university offers precious opportunities for us to meet our lifelong friends.
Although his remarks are provocative, they 一理ある (are true in some respects).
Ex3 (On the Internet)
A: Why did you quit Twitter?
B: Cause it’s a waste of time.
A: 一理ある (Yeah you’re right)
Synonyms: 確かに (That’s true; You’ve got a point)
Synonyms in English: true in a way, true in some respects
Related words and phrases: 一 (one), 理(rationale, reason), ある (exist)