Summarized from Mr. Wikipedia for you by me:
Privishing refers to the process of technically publishing a book without really publishing it. It’s printed in such small numbers or with such lack of marketing, advertising /sales support/enthusiasm that the book may as well never have been published in the first place. Said book is virtually impossible to obtain through normal channels, often can’t be special-ordered and is very seldom reprinted.
In short, when a book is privished it’s "killed".
Technical adherence to the terms of the publishing contract yes, but no more. Print-runs and marketing/sales budgets of controversial books will be slashed if a Publisher feels that promoting it will adversely affect business.
Privishing can be seen as a form of censorship too, particularly when it occurs in response to pressure from an irate author, or whomever. Threats of legal action or complications involving parent companies are often cited as reasons for privishing. Privishing also happens when there’s a true lack of interest in a title, a change of direction for the publisher or in some cases where a key editor on the title resigns.
Because only a few books a year can be big successes publishers typically put their money and efforts where they’ll get the most back. If a book doesn’t generate early interest from key outlets, such as large department stores, then it will probably be left to flap in the wind.
The size of an advance paid for a book can also determine whether or not a book is privished. If a publisher sinks big coin into a book then it’s likely to put at least enough money into sales/ promotion to at minimum earn the bread back. Books with low advances are killed with little publisher pain.
Privishing is a self-fulfilling prophesy: as initial sales efforts are subtly reduced, interest wains, marketing and sales efforts lose their priority. The author has no way to independently verify this and so can’t know if his title has been purposefully killed or not.
Books have a very short time frame during which to make an impact with the buying public, it’s almost impossible to determine the source of failure for a particular title, as a result, privishing has rarely been prosecuted in court.