expression, noun
love for one another; being in love
Describes a mutual romantic relationship; often used for couples who are clearly more than friends.
あの二人はいい仲らしい。
Those two seem to be in love with each other.
彼らはもういい仲だ。
They are already in a romantic relationship.
恋人 is a concrete noun meaning 'lover' or 'boyfriend/girlfriend', while いい仲 describes the state of being in a mutual romantic relationship.
Combination of いい (good) and 仲 (relationship), literally 'good relationship', used idiomatically to mean a romantic involvement.