Sentence:  Another good reason to use first-class mail is the fact that it will often be forwarded or, if need be, returned to the sender if the address is bad. (In the industry, this is called a “<WSD>nixie</WSD>.”) It is important to keep an eye on the <WSD>nixie</WSD> rate if you are using first-class mail. 
Meaning of the word marked with <WSD> tags:  A piece of mail returned as undeliverable.