It's a tool used for abuse that is meant to cause the victim to start doubting reality and thus, themselves, through subversive techniques. Generally, it's used to change the definitions of terms and is typically used by the alt-right.
Gaslighting is a form of psychological manipulation that seeks to sow seeds of doubt in a targeted individual or in members of a targeted group, making them question their own memory, perception, and sanity. Using persistent denial, misdirection, contradiction, and lying, it attempts to destabilize the victim and delegitimize the victim's belief.[1][2]
So basically anything you don't agree with.
Here's a gaslighter for those who are actually ignorant of what is being discussed here. Look at him go.
You gonna get on your alt accounts too? ---
"I don't think anyone seriously thinks that Trump supporters orgasm when they see racism in the news." - Me, reassuring Ammonitida