A relation, relationship, or connection between people or things (e.g., family relations, causal relations).
彼と私は仕事上の関係がある。
He and I have a business relationship.
二つの事件の間に関係があるかどうか調べている。
They're investigating whether there's a connection between the two incidents.
家族関係が複雑で、調査に時間がかかった。
The family relationships were complicated, and the investigation took time.
上司との関係を良くするために挨拶を欠かさない。
I never skip greeting my boss to maintain a good relationship with them.
Participation, involvement, or concern — being connected or implicated in something (often used about people involved in an event).
事件に関係のある人物が数名特定された。
A few people involved in the incident were identified.
その問題には直接関係していないが、影響は受ける。
I'm not directly involved in that issue, but I will be affected.
学生会の活動に関係する書類を提出してください。
Please submit the documents related to the student council activities.
Influence or effect — how one factor affects another.
睡眠不足は学業の成績に深い関係がある。
Lack of sleep is closely related to academic performance.
気温の変化が作物の生育にどのような関係を持つか調べる。
We study how temperature changes affect crop growth.
価格と需要の関係をグラフにする。
Plot the relationship between price and demand on a graph.
Sexual relations — referring to sexual or intimate relations between people (context-sensitive).
彼らは過去に性的関係があったと報じられた。
It was reported that they had a sexual relationship in the past.
不倫や性的関係の問題は職場で大きなトラブルになることがある。
Affairs or sexual relationship issues can cause major trouble in the workplace.
Used as a suffix meaning 'related to' or 'connected with' (e.g., —related: legal-related, environment-related).
法律関係の書籍を図書館で探した。
I looked for law-related books in the library.
環境関係のセミナーに参加する予定です。
I plan to attend an environment-related seminar.