Cheap is an interesting subject. Many people use it incorrectly. Usually people call it cheap when they did something stupid and got punished for it or when they're being outskilled.
The real definition of cheap is a broken game mechanic. For example, some people might say Fox's shine is cheap, but it really isn't. What may be cheap is f throw to off-stage shine leading in a 0% kill. That's cheap.
Disclaimer: I was at a professional level in Halo: CE. I know what I'm talking about.
A good example is also in Halo: CE. The ghost, was cheap. It was completely broken in a few situations. Mainly when you're on foot or in a vehicle. Which is pretty much 9/10's of the game. A skilled ghost pilot in a small game couldn't really die. A skilled ghost pilot will 100% kill you in a tank. A skilled ghost pilot will 100% kill you if you're on foot and can't find cover immediately.
As I said, "cheap" is an aspect of the game itself. If the game has flaws that allow something to be exploited that removes skill from the equation, it's "cheap". A very obvious example would be if Melee had a "secret button combo" where you won automatically. That would be hella cheap.
As Mic_128 said, being thrown after you've just spawned to an edge-gaurd spike/meteor isn't cheap, it's your fault. You can wiggle out of grabs almost immediatly at 0% or if they grab, attack, and then try to throw you can get out of it before they throw 100% of the time. It's also your fault for DI'ing incorrectly, and if they meteor'd you, it's your fault for not meteor cancelling at the right time.
There are a lot of games with certain broken game mechanics. I just wanted to throw this clarification out there.
And M3D got it perfect. Cheap doesn't exist. Broken game mechanics that make the game unplayable/not worthy to be played are what exist. On a side note, this is why games that can be played competitively are so beautiful. They are so well made, so precise, that it is a work of art. Not to mention that games that are "deep" can be played with such creativity and originality that the players themselves are practically painters.