Why is the ninth month called September?
1560s, from residue + -al (1), or from French résiduel, from Latin residuum.
1550s, in mathematics, from residual (adj.) or from residue + -al (2). Residuals "royalties" attested by 1960.
residual re·sid·u·al (rĭ-zĭj'ōō-əl)adj.
Of, relating to, or characteristic of a residue.
Remaining as a residue.