The species problem persists due to the lack of a universal, empirically testable definition across sexual, asexual, and borderline life. We propose a genetic compatibility framework: a species is the largest group genetically compatible for reproduction, evidenced by offspring production regardless of viability or fertility, or by a shared clonal lineage with high genetic similarity. This criterion explicitly applies across the primary kingdoms of life: single-celled organisms, Fungi, Plants, and Animals. Applications to diverse taxa demonstrate clear and consistent species boundaries, resolving longstanding classification inconsistencies.