Being in a romantic relationship with someone; dating. Equivalent to 'dating' or 'seeing someone' in everyday speech.
あの二人は長い間交際中だと聞いた。
I heard those two have been dating for a long time.
彼女は今交際中だから、紹介話は断られた。
She is currently in a relationship, so my introduction offer was turned down.
交際中のカップルは互いの気持ちを尊重することが大切だ。
Couples who are dating should respect each other's feelings.
高校時代から交際中の彼らは、来年結婚する予定だ。
They, who have been dating since high school, plan to marry next year.