What is a "no contest" in Boxing?

Question

When you hear the records of some boxers, you see they have either 1 or 2 NC or No contest in their record.

Answer 1

Usually a No Contest means that there has been some sort of accident early on in the fight (first few rounds) like say an accidental clash of heads that has caused one of the fighters to get cut that bad that the fight cant continue this would then be called a no contest, however if it happens later in the fight it would go to the scorecards.

Answer 2

What the guys said above, also it has been used to change prior results when a fighter fails the post fight medical screening and turns up positive for substance.  i.e. James Toney vs. John Ruiz.

Answer 3

In some states if a fight is stopped on an accidental foul before 4 rounds are completed the fight will be considered a no contest.  

If the fight goes past 4 rounds and is stopped due to an accidental foul the fight will go to the score cards for a technical decision. (TD)

Also sometimes if  a fighter is found after the fight to be on some sort of steroid or has used some sort of illegal substance the fight will be re declared a No contest.

A no contest does not count on a record, its still there but the rankings or titles are not affected.

Mike Tyson vs Orilin Norris was called a no contest a few days after the fight was over, and the fight was changed from a TKO for Tyson to a No Contest when Tyson tested positive for marijuana.