In comics, the term "ongoing series" is used in contrast to limited series (a series intended to end after a certain number of issues), a one shot (a comic book which is not a part of an ongoing series), a graphic novel, or a trade paperback. However, a series of graphic novels may be considered ongoing as well.
The term may informally refer to a finite series if the number of issues is predetermined.
An ongoing series is traditionally published on a fixed schedule, typically monthly. However, many factors can cause an issue to be published late. In the past, the schedule was often maintained with the use of fill-in issues (usually by a different creative team, sometimes hurting quality), but increasingly the practice has been to simply delay publication.
When an ongoing series ceases to be published because the story has ended, it may be called "finished." If it ceases to be published because of low sales, editorial decisions, publisher bankruptcy, or other reasons, it is "cancelled." (An ending might be written for the last issues of a cancelled series, or the series may simply disappear without warning and never return).
If a series ceases to be published, but may be published again, it is called "on hiatus." Many series are placed "on hiatus" but do not return even after several years.
For series that are creator owned, the copyright holder has the option of approaching other publishers to see if they would be open to resuming the title under their imprint. For instance, Usagi Yojimbo has had four consecutive publishers.