Abandonment occurs when the trademark owner stops using the mark with an intention to not resume using it. Use of a mark in this context means the bona fide use of the mark made in the ordinary course of trade, and not made merely to reserve a right in a mark.

Non-use for three consecutive years is prima facie evidence of abandonment. The trademark owner in such instance has the ability to rebut the presumption of abandonment by submitting evidence of an intention to retain their trademark rights. This may include

The very fact that the presumption of abandonment doesn't kick in until after three years of non-use seems to imply that non-use extending less than three years is not abandonment. While, trademark lawyers may be destined to argue this point, it may not accurately reflect the law on this matter.  Ultimately what matters is the bona fide intent to preserve the trademark and resume using it in commerce, regardless of how much time has lapsed. It's just that after the three years, the burden of proof shifts to the owner to prove intent not to abandon. Common sense also dictates that the longer a trademark owners waits to resume use, the harder it will be to prove their intention to resume use was there all along.     

Furthermore, a trademark can become abandoned inadvertently when the trademark owner causes the mark to become the generic name for the goods or services on or in connection with which it is used - or otherwise allows the mark to lose its significance as a source identifyer.  This may include consenting to a third party's confusingly similar use of the mark, although some jurisdictions look at this consent issue more stringently than others.

If abandonment occurs, all rights in the mark cease.  Some jurisdictions may entertain the argument that residual goodwill in a zombie mark allows for resurrection of some form of trademark rights following abandonment.