On jump scares - here is why I don't consider them or other tension based things horror.
If I'm playing a stealthy level of a shooting game, I'm going to be tense. And when I pop around a corner and see a guard, I might jump or be startled, or I might be tense or nervous about moving into the unknown when there are patrols around, but that doesn't make the mandatory COD stealth missions or Dishonoured HORROR. A tense cat and mouse moment in a movie may have me feel tension and even fear, but that doesn't make it horror, like there are scenes like that in Disney movies.
We have to separate video games and movies now.
For movies: gore isn't scary. A bloody massacre isn't scary. Kill Bill, Inglorious bastards, these aren't horror films. Western "horror" in general typically kind of sucks at being actual "horror", because they lack a certain element of subtly the genre demands (like a crime drama where the criminal wears "I'm the killer!" T-shirts in every shot). I would categorize movies where gore, adrenaline and startling are the three main methods of interacting with the audience as thrillers. They get your heart beating, but they don't scare you, do they? Not really. You don't watch Texas chainsaw massacre and then have trouble going to sleep because what you saw leaves you shaking at the thought of a dark room. You have trouble going to sleep because you saw a hillbilly chainsaw murder somebody in a high octane action movie style chase.
It's called horror because it's supposed to horrify. Movies that play into your fears, fears of elements, nature, people, supernatural, whatever, and remind you that if these elements turned on you, there would be nothing you could do.
Horror movies remind us that we're not afraid because we're alone in the dark - we're afraid because we might not be as alone as we think we are.
On to video games.
As a general rule of thumb, I don't consider any game where you can respond to the horrors you see with a shotgun blast to be a "horror" game. There's nothing scary about Deadspace. You're a walking fucking tank. When a monster goes OOGABOOGA you rip off its legs and beat it to death with them. YOU are the scariest thing in Deadspace.
There are almost no games where you have a full arsenal that I could consider horror. Because there's nothing horrifying about carrying a weapon and having the skill set to fuck up everything that comes at you. Is resistance horror? No. Sure dark hallways are spooky but in Deadspace I have a fucking giant gun that can kill super alien monsters. In a game like Asylum where my only weapon is running and screaming like a little girl, the fear I feel looking down a simple hallway is fucking intense.
Play PT. it was supposed to be a demo for a horror game that was never finished but the demo is nightmare inducing. It creates a fear so visceral and intense it will keep you up for weeks, and it accomplishes it with one single dark hallway and occasionally a bathroom.
Alien Isolation started great. The tense cat and mouse fights between you and the alien were terrifying, because you knew nothing you had could hurt it, and if the Alien caught you, you were fucked. Plus the helpful androids turning murderous was terrifying too. The game nosedived once you got the flamethrower and the badass alien turned into a nuisance rather than something frightening.
Silent Hill 2 was amazing because it delved into the true depths of psychological horror. It was great because you never really knew if the monsters you were running from were really there, or just figments of your imagination, and there was this unsettling feeling that it didn't matter either way because if you really were crazy, then they are real enough to hurt you.
It's like when you're alone at night and you hear noises, and there's a stomach churning dread about whether what you heard was real, or all inside your head, and if it makes a difference. It's the kind of fear that makes you run up the stairs when you turn the light off, because even if your eyes say nothing is there, you can sense something creeping up your spine.
It's the feeling in certain amnesia and Alan Wake games that you don't WANT to look behind you because you're afraid of what's gonna be there, so fuck it, just run as far as you can. That's scary.
Something that makes you reflexively put 80 bullets of your explosive assault rifle into its inhuman face isn't scary because you have the means to proactively put down the abyssal horrors like Rambo with a ghost busters proton laser.