Is it merely interceptions, or is it having such a reputation that the opposition won't throw in your direction?
Could it be having the assignment of defending the other team's No. 1 receiver on every snap, no matter where he lines up on the field? Or is it a combination of everything?
Steelers cornerback Ike Taylor would like to know the definition, because if anybody in the NFL should be called a shutdown corner, he fits that description.
Through three games, Taylor has allowed three catches for 18 yards in the 20 times he has been targeted while being asked to cover Lee Evans, Mike Williams and Reggie Wayne - all No. 1 receivers.
Taylor will draw another top receiver Sunday when he will be matched against Houston's Andre Johnson.
"I don't see anybody better than Ike," Steelers receiver Mike Wallace said. "When it comes to covering, I'd put my money on him any day. People are sleeping on Ike Taylor."
Taylor blanketed Wayne last Sunday and allowed only an 11-yard catch out of 10 targets to Wayne, while Taylor was covering him.
"He is one of (the best in the league)," Steelers quarterback Ben Roethlisberger said. "He is fast, he is strong, he is big and it really makes it tough. Guys are more recognizable than him name-wise, but as far as playing, I think he is one of the best."
What makes Taylor unique is that he covers the other team's top receiver no matter where that player lines up. And Taylor usually is playing press-man coverage with no help.
That alone makes Steelers safety Ryan Clark think Taylor is better than the preeminent shutdown corner in the league — Nnamdi Asomugha — and should be mentioned in the same breath as the Jets' Darrelle Revis.
"Nnamdi's stats compared to Ike's stats over the past three years aren't even close when it comes to production," Clark said. "It is about perception. Nnamdi wasn't playing cover-zero every play in Oakland, but you would think that because that's the perception."
Asomugha was targeted only 29 times last year while playing in Oakland, then signed a five-year, $60 million deal with the Eagles.
"Once people talk about you in a certain way, then that becomes your perception," Clark said. "Perception becomes reality, and then you go to Pro Bowls."
The perception about Taylor, who signed a four-year, $28 million deal in the summer, is that he is far from being considered a shutdown corner and maybe even further away from being a Pro Bowler.
A recent ESPN.com poll asked experts to rank the top cornerbacks in the league on a scale of 1 to 10. Taylor not only didn't make the top 10, he didn't receive a vote - 20 other corners in the league got at least one vote.
"I'll let you all deal with the stats. I just take it one play at a time," Taylor said. "It is just being consistent week-in and week-out for me."
Taylor was bypassed the first two weeks; he was targeted only seven times. Against the Colts he was thrown at 13 times, with the only two catches being quick-hitting pick plays at the line of scrimmage.
There's still that one issue when it comes to Taylor - his hands.
"The thing that keeps him from being mentioned with those guys is just catching the ball," Clark said.
Taylor had only two interceptions last year and never has had more than three in a season. He has only 11 picks in 128 career games.
Then again, neither Asomugha nor Revis had an interception last year and both made the Pro Bowl.
"I would love (a Pro Bowl) on my resume," Taylor said. "I'm going to get it — just don't know when."