Why is the ninth month called September?
mid-14c., from Old French difference (12c.) "difference, distinction; argument, dispute," from Latin differentia "diversity, difference," from differentem (nominative differens), present participle of differre "to set apart" (see differ). Sense of "a quarrel" first attested late 14c. Colloquial phrase what's the diff? first recorded 1896.
difference dif·fer·ence (dĭf'ər-əns, dĭf'rəns)
The magnitude or degree by which one quantity differs from another of the same kind.