The Brief: A trigger is something that may invoke a past traumatic memory, typically related to PTSD.

DEEP DIVE
A trigger is something that may invoke a past traumatic memory, typically related to PTSD. Verb forms include “to trigger” and “to be triggered”. “Triggering” is the adjective form.
More broadly, the word trigger may be used to signify that something causes a reaction.
Online, the term “triggered” or #triggered is often used to mock people for being overly sensitive, particularly in regards to opinions they disagree with.
Triggered can also be used lightly not specifically referring to PTSD, to mean “bothered.”
I just woke up from a fucking nightmare where I couldn’t graduate high school omfg I’m triggered
— James Charles (@jamescharles) July 9, 2018
Trigger Warnings are used to identify potentially triggering content so that those who may be triggered can censor it if needed.