/ob/ prefix Obligatory. A pieCe of netiquette aCknowledging that the author has been straying from the newsgroup' s Charter topiC. For example, if a posting in alt.sex is a response to a part of someone else' s posting that has nothing partiCularly to do with sex, the author may append "ObSex" (or "Obsex") and toss off a question or vignette about some unusual erotiC aCt. It is Considered a sign of great winnitude when one' s Obs are more interesting than other people' s whole postings. [Jargon File]