A pearlescent liquid that turns crafted items back into their ingredients or into something new entirely — and it only exists because of one biome, found on the same side of the map as your Jungle.
Shimmer is a lavender, soap-like liquid that either decrafts an item back into the materials it was made from, or transmutes it into a different item outright — and it's also the only liquid that will drag your character down through solid blocks instead of letting you swim. It was added in the 1.4.4 "Labor of Love" update, and every world made before that patch simply doesn't have it — you need a fresh world to get access.
Shimmer only occurs naturally inside a mini-biome called the Aether. It generates once per world, in the Cavern layer, and always on the same half of the map as your Jungle — so the first thing to check isn't a map marker, it's which side the Jungle spawned on. From there, head to the ocean on that same side and start digging down: the Aether tends to sit under that stretch of coastline, deep enough that you'll already be in Cavern-layer stone by the time you reach it. It's easy to recognize once you're close, because the background switches to a distinct starfield unlike anything else underground.
There's no shortcut that skips the digging — no NPC sells a locator, and no ore vein points the way. If a dig down the Jungle-side coast turns up nothing, the biome is usually just a bit further along the shoreline or slightly deeper than expected.
Drop a crafted item into Shimmer and one of two things happens. Most items that were assembled at a crafting station decraft: they break apart into the ingredients that went into them, which then float back up to the surface for you to scoop up. That makes Shimmer a free way to walk back a bad craft — turn a weapon you don't want back into bars and reagents instead of losing them.
A smaller set of items don't decraft at all — they transmute into something else entirely, one specific item for one specific item, no coins or crafting station involved. The Rod of Discord is the clearest example: dunk it in Shimmer after Moon Lord is dead and it comes back out as the Rod of Harmony, a strictly better teleport rod that drops the Chaos State debuff its predecessor inflicts. Both directions of that swap work — you can shimmer a Rod of Harmony back into a Rod of Discord too, for reasons that mostly come down to preference. A wide range of other gear works the same way, from tools to accessories, and the items database is the place to check whether something specific has a Shimmer recipe before you throw it in and hope.
Submerge your character and you don't drown — you get the Shimmering debuff instead, which locks you out of moving your character normally, using items, and taking damage from anything short of a boss, while you sink slowly through every solid block beneath you. You keep sinking until you hit an open pocket at least a couple of blocks tall, or you fall all the way to the Underworld if nothing stops you first. It's disorienting the first time, since there's no way to cancel it early on foot, and if you never press a direction the game will eventually force a teleport rather than leave you stuck.
The one piece of gear that changes this is the Chromatic Cloak, made by shimmering a Star Cloak: instead of phasing automatically the moment you touch Shimmer, it lets you choose when to sink by holding a direction, which is the difference between using Shimmer on purpose and falling into it by accident.
Shimmer can't be scooped up with an ordinary bucket — it's one of the few liquids in the game that specifically resists that. The practical way to move it is with pumps and a hose network, which is how most Shimmer farms built away from the Aether get their supply. Alternatively, once Moon Lord is down, you can craft a Bottomless Shimmer Bucket at the Ancient Manipulator from a Bottomless Water Bucket and Luminite Bars, giving you an unlimited, portable source you can place anywhere — no plumbing required, and no more trips back to the Aether.
A handful of upgrades exist purely as a Shimmer recipe, with no drop table or shop entry standing in as an alternative. The Rod of Harmony is the standout, since the debuff-free version of the Rod of Discord simply isn't sold or dropped anywhere. The rest tend to be quality-of-life swaps rather than combat gear — things like turning a Clentaminator into the endgame Terraformer, or an Enchanted Sundial into its night-time counterpart — small item-for-item trades that exist only because someone was willing to throw the original into a pool of pearlescent liquid and see what floated back up.