What is cheating?

If you’re an offensive lineman and you find a way to illegally hold a defensive tackle on every play without officials noticing, is it cheating?

If you’re a coach and you’ve been deflating footballs so your receivers can make catches more easily, is it cheating if no one knows about it?

Most of us would say the first case is the lesser transgression, by far, but in reality, both are examples of the same thing: trying to gain an unfair competitive advantage. Or, as it’s commonly called, cheating.

(I don’t want to get into a discussion about what holding is in the NFL. We’d have an easier time agreeing on the meaning of life.)

Sports is a morality play but with lots of gray areas in which the good guys aren’t always distinguishable from the villains. Ever since our hairy ancestors starting corking their clubs for the rock-hitting contest, there has been cheating in sports.

The NFL is investigating whether the Patriots used deflated footballs during their 45-7 victory over the Colts in the AFC Championship Game. According to ESPN, New England underinflated 11 of its 12 game balls Sunday. Patriots coach Bill Belichick initially said he wasn’t aware of the issue, though this is the same guy who once had to pay a $500,000 fine for spying on another team.

Deflating balls to make them softer for receivers — cheating or competing? As late Supreme Court justice Potter Stewart famously said of pornography, “I know it when I see it.’’ If the allegations are true, Belichick is naked here, with no hoodie in sight. Sorry for the imagery.

But what I call cheating, you might call using everything at one’s disposal. I think using steroids is unfair, immoral and dangerous, but some people have no problem with them and believe athletes should be able to take drugs to get stronger and healthier. Cheating has been going on forever — I can’t tell you how many times I’ve heard that counterargument to the scourge of steroids. File that under “All’s fair in love and war.’’

Pete Rose recently said that Roger Clemens and Barry Bonds belong in the Baseball Hall of Fame, never mind the widespread accusations of steroid use against them. That might seem like an arsonist defending two burglars, but Rose speaks for a lot of Americans.

Yet I’d ask the pro-steroids crowd: Where do we draw the line? Should prostheses be allowed, a la Olympic sprinter Oscar Pistorius? There is little doubt that technology one day will surpass, at a sprint, the limbs we were given at birth. Bionic running blades will be an unfair competitive advantage, and there surely are people who would give an arm and a leg to win a gold medal. But taking the anti-prostheses argument to the extreme, corrective eyewear and Tommy John surgery are also unfair competitive advantages.

Did the 1985 Bears cheat by gobbling down non-prescribed painkillers, as some players from the team say they did? If it allowed them to play at levels they wouldn’t have been able to play at otherwise, then the answer would have to be yes. But what if everyone in the NFL were doing the same thing at the time? If everyone is cheating, there’s no competitive advantage, correct?

I think the ultimate answer is what our moms used to say: Just because everybody is doing it doesn’t make it right. That’s the best way to dismiss Lance Armstrong, who turned out to be more rolling medicine cabinet than professional cyclist. He said every other elite competitor was using drugs to make it through the Tour de France. Very, very not OK, Lance, especially when you’ve inspired millions with your comeback from cancer.

Most of us would agree that stealing intellectual property from another business is a crime. But that’s what Belichick tried to do in 2007, when he had an assistant spy on the Jets in order to get their defensive signals. Cheating, right? But what about stealing signs in baseball? Gamesmanship, we’re told — a part of the game going way back.

Some things strike us as more wrong than others. Deflating balls seems more serious than, say, an offensive lineman holding an opponent. The spirit of the game, and all that.

It’s not cheating if you don’t get caught? Yeah, it still is, but there are venial sins and cardinal sins. Most of the time we can tell the difference.