Is it ensure, insure, or assure?
"coming in regular succession," 1840, from series + -al (1); popularized in reference to Dickens' novels, published one part at a time in periodicals (as opposed to all at once in a book). Found to be a useful word and given wide application. Serial number, indicating position in a series, first recorded 1866, originally of papers, packages, etc.; of soldiers from 1918. Serial killer is first attested 1981 (in relation to John Wayne Gacy and Ted Bundy), though serial had been used in connection with murders since the early 1960s. Related: Serially.
1846, from serial (adj.).