In the universe of the Original Series, a centon is a Colonial measurement of time.
The term is analogous to a minute. The plural form is "centons."
One (1) centon is equal to one hundred (100) microns.
In "Murder on the Rising Star", Boomer used the phrase "Just a centon," the Colonial equivalent of "Just a minute."
In "Saga of a Star World", it appears that centon meant something closer to a week. ( Secton came into use as the Colonial equivalent for week in "The Living Legend, Part I ) In the pilot, Apollo told the Quorum of the Twelve that the intended route to Carillon would have taken "taken us centons of our way." The use of two other time units - millicenton and microcenton - seem to have been based off of this original definition of the centon as a week, especially when you consider the metric prefixes of "milli" (thousand) and "micro" (million). Both terms were used almost exclusively in the first two episodes.
Beginning with "Lost Planet of the Gods, Part I", use of the term centon was consistent in the series. In it, Lucifer told Baltar that the outpost was 1.5 hectares from the Galactica and that they could catch the Galactica at light speed "within a centon." According to the Writer's Guide, a hectar is 9 million miles, or roughly a light minute.
See also: Units of Measure (TOS)