"Fridging" is shorthand for the "Women In Refrigerators" trope, defined by a female character being brutalized, sexually assaulted or murdered in order to spur a male character to action or develop their character. It's particularly prevalent in the superhero genre, which is where the trope name comes from, coined by comics writer Gail Simone in 1999, and taken from 1994's Green Lantern #54, in which Kyle Rayner returns home to find his girlfriend's dead body stuffed inside his refrigerator.
Not only does the trope reduce female characters to plot devices, it also supports the sexist stereotype that women -- by virtue of gender alone -- are vulnerable, punchable damsels. And, if you think we're exaggerating about just how prevalent it is, check out this exhaustive (and open-ended) list.
You could argue that Thanos reversing time in order to re-kill Vision himself and recover the Mind Stone balances out what he does to Gamora; after all, Thanos is wiling to kill both female and male characters in his quest for the Stones, right? The key difference is that Vision's second death doesn't benefit Thanos' character in the way that his slaying of Gamora does. Where Vision's death confirms the Titan's monstrosity, Gamora's death serves to humanize him. He gains a huge amount from it -- not just the Soul Stone, but an actual soul as a father forced to sacrifice the only thing he truly loves in order to fulfill his life's work. It's an undeniably heartbreaking moment for them both, as Gamora realizes that her father's love for her is real at the very moment that she learns that the same love has sealed her fate.
Similarly, as an audience, we can appreciate the Shakespearian tragedy of this moment at the same time that we realize that Gamora -- who Thanos himself described as "the strongest woman in the galaxy" -- ends a three-movie arc having served a purpose that is now cheaply singular. Daughter of Thanos she may be, but in the end, her life just couldn't measure up to an inanimate, glowing rock. Her death will likely be non-permanent (as will most everyone else's) but a resurrection -- and possibly revenge mission -- won't mean the fridging never happened -- just that it didn't stick.
When writers Stephen McFreely, Christopher Markus and the Russo brothers decided to throw out Thanos' usual motive (romancing the sexy, skeletal embodiment of Death) they knew that something else would be needed to create audience sympathy for the Mad Titan. Building on a relationship with an established character rather than insert a brand new one makes total sense, but, by choosing Gamora, they wrote themselves into a fridging corner.
This is the point at which two problematic tropes clash: one being the aforementioned fridging, and the other being the idea that men aren't "naturally" empathetic. The perceived vulnerability of women means female characters lose our sympathy, whereas male ones have to earn it. Thanos is, of course, a genocidal maniac, no matter how zen he is about it, so his villainy already puts him at an even higher disadvantage in the empathy stakes. (Plus, he traumatized the Hulk, and we're pretty fond of the big, green lug.) But, by shedding a few purple tears, Thanos does indeed earn our sympathy. After all, when was the last time you saw a supervillain cry so mournfully?
It doesn't help either, where gender is concerned, that the only two female characters who have any kind of significant roles in the film play them supportively; Scarlet Witch is the weepy girlfriend and Gamora is the weepy girlfriend, kidnapped damsel and sacrificial lamb. Thank God, then, for the small -- and probably intentional -- counterbalance of Okoye and Black Widow's dry-eyed (and hard-eyed) stares as they briefly steal the macho spotlight to take down Proxima Midnight. It's worth noting as well that, despite being tied to Vision constantly, Scarlet Witch does get her hands mystically dirty on the battlefield, even holding her own against Thanos while he's armed with a one-stone-short-of-a-full-set Infinity Gauntlet.
Her valiant effort, however, does nothing to change the fact that in Avengers: Infinity War, Thanos wins big. Not just narratively, but he kind of wins our hearts too. It's just a real, real shame that the writers couldn't come up with a way to do that that didn't stop Gamora's from beating.
Directed by Joe and Anthony Russo, Avengers: Infinity War stars Robert Downey Jr., Chris Evans, Chris Hemsworth, Mark Ruffalo, Jeremy Renner, Scarlett Johansson, Paul Bettany, Anthony Mackie, Paul Rudd, Elizabeth Olsen, Tom Holland, Benedict Cumberbatch, Chadwick Boseman, Chris Pratt, Zoe Saldana, Dave Bautista, Bradley Cooper, Vin Diesel, Tom Hiddleston and Josh Brolin. In theaters now.