noun
serious relationship; dating with marriage in mind
Used especially in contexts like matchmaking and dating apps to describe a relationship where both parties are looking for a marriage partner, not casual dating.
結婚相談所では、真剣交際を希望する人が多い。
At marriage agencies, many people are looking for a serious relationship with marriage in mind.
彼とは真剣交際をしているので、将来のことも話し合っている。
I'm in a serious relationship with him, so we also discuss our future together.
Compound of 真剣 (serious, earnest) and 交際 (association, dating). The term likely emerged in the context of modern matchmaking services to distinguish marriage-minded dating from casual relationships.