Battle Ballgown

A dress that is lightly to heavily armored, and thus would fit in either a social gala or an epic melee. The form of the dress can range from a fancy dress accessorized with armor and/or weapons (even if it also has typical dress trimmings), to basically a suit of armor with a skirt.

The point of this dress seems to be a combination of Rule of Glamorous and Rule of Cool, as such an outfit will almost always be Impossibly Cool Clothes. Armored skirts were historically used (and mostly for men at that) as they allowed waist protection to be maximized without obstructing the legs but they never got as large as many of the examples in this article as the sheer weight would restrict mobility anyway.

Naturally the ladies wearing these kinds of dresses are almost always Action Girls: Ladies Of War and Badass Princesses or other tropes related to Silk Hiding Steel (literally inverted in this case). If the dress is on the fancy end, it overlaps with Kicking Ass in All Her Finery. If any other girl ends up wearing this, expect her to fall over almost immediately, if she can even move.

A Sub-Trope of Pimped-Out Dress and Costume Porn.

Compare Stylish Protection Gear, Scary Impractical Armor, Powered Armor, Hot Chick in a Badass Suit.

Contrast Chainmail Bikini, Stripperiffic.

Remember that this is an armored dress, not simply a fancy dress worn in battle.

Examples of Battle Ballgown include:

Anime and Manga

In Soul Eater this is initially averted when Maka and the rest of the cast change out of their formal gear during a Ballroom Blitz. However later, Maka converts the Black Blood into Maka's dress/armour can be seen armour in the shape of the ballgown she appears in inside Soul's mind while they're linked while they fight Arachne.
Some of the Barrier Jackets (customizable magical armor suits) worn by female mages from the Magical Girl Lyrical Nanoha series. Take Signum, for example.
Fairy Tail Erza's standard outfit certainly applies, and there's no question about this one
- And then there's her strongest armor
Subaru wears this in-game in .hack//Sign.
- As do Terajima Ryoko in the quadilogy game, being Subaru's pallette swap. Ditto Kaochin.
Asuna shows up wearing this in several Negima artworks soon after getting her artifact. Before long, she was wearing them in the series proper.
- See also the Adriane Knights in the Magic World, and thus by extension Yue.
A Super Robot Power Armor version exist in Infinite Stratos in form of Uchigane, a Red Shirt training unit of sort. The shape is reminiscent of old-style Samurai Armor, but it has extremely feminine design (with open top for Fan Service reason). It's also listed as Defense/ Balance type.
Magic Knight Rayearth
Saber from Fate Stay Night and Fate/Zero.

Art

Paintings of Joan of Arc often show her in armor with a skirt.

Live-Action Films

In the Walden Media film version of The Lion, the Witch and the Wardrobe, the White Witch wears a breastplate and a chainmail dress to battle, along with Aslan's shorn mane.
- The film version of Prince Caspian has Susan actively participating in more battles than the book, wearing chainmail, leather, and a long skirt, and making her a fully-fledged Lady of War.
Mary of Guise is shown wearing a fancy dress appropriate for a dowager queen under a sensible breastplate when she taunts the English after a battle in Elizabeth.

Literature

In The Wheel of Time, Nynaeve, in a moment of insecurity in Tel'aran'rhiod, wears an impossible dress made of armor plating.
Honor Harrington's tunics and tabards are made of a material with (limited) anti-ballistic properties. Mostly because Nimitz likes to ride on her shoulder and his claws would shred any lesser material. It has saved her life on one occasion, and her bodyguard's comfort in knowing she wears them is repeatedly mentioned.
In the "Mistborn" series by Brandon Sanderson, the main character, Vin, is fitted for ball gowns that designed to allow for a range of motion, silent movement, and with hidden pockets. She still prefers not to have to fight in them, since they tend to wind up destroyed if she does, but being Crazy Prepared she's just more comfortable if she has the option open.

Live-Action TV

Xena's wedding dress in "Soul Possession" was both stylish, practical and pretty damn cool.
- Also this little number from "Return of The Valkyrie".

Tabletop Games

In the art for the EverQuest pen & paper RPG book, the studded leather armor is drawn like an armored dress.

Video Games

The hoplite class in Etrian Odyssey III.
- The Princess class has a lighter armor dress, with tiaras, frills, and fur trim, on the one hand, but metal boots, gauntlets, and one dress with a breastplate, on the other hand.
Saber of Fate/stay night has a regular Pimped-Out Dress that she wears armor over when she heads into battle.
Chris Lightfellow of Suikoden III
Vanillaware often uses this design for female fighters, especially those with heavy armor.
- The eponymous heroine of Princess Crown.
  - Homaged with Princess Eclair in La Pucelle.
- Gwendolyn, from Odin Sphere. Her supposed battle dress is fabulous, having loads of Fluffy Fashion Feathers and ruffles.
- Appears in Grand Knights History.
Soul Calibur IV, Ashlotte's outfit.
- Soul Calibur III offered this as an option for female created fighters, whether mix-and-match or complete armoured outfit.
Wild ARMs XF has Alexia, who wears a Pimped-Out Dress accessorized with plate mail into battle.
All 6 contestants in the Princess Waltz, whose dresses (being comprised of magic and being of varying designs) are their armor. Particular note goes to Angela, who flawlessly accents her dress with a breastplate.
Samurai Shodown's Charlotte goes tank on top, skirt on bottom.
Touhou Project has Yukari Yakumo. While her dress LOOKS normal, Hong Meiling's victory quote against her implies that Yukari has some sort of armor underneath her gown, saying "I felt some odd resistance. What are you wearing underneath your clothes?"
- Being both a supernatural being and a Reality Warper, it could very well be that Yukari's skin and/or bones are incredibly tough, so much so that it feels as hard as armor.
Most of the armor for the female avatars in the Monster Hunter games fit this description.
Tactics Ogre, at least in the 2011 re-imagining of Let Us Cling Together features this geddup for its female knights.
Mabinogi has Leminia's Holy Moon Armor for female characters. It's pretty much the male version with a skirt and is the most expensive armor buyable from NPCs.
In Chrono Trigger, the Prism Dress is the strongest armour, and can only be worn by women.
Lenneth, Hrist, and Silmeria from the Valkyrie Profile series all have outfits that fit this trope.
Special mention goes to Dark Souls. While there are "gown"-looking armor in the game, there are usually no reason to equip it except for the (now-patched and nerfed) Antiquated Skirt, which has disproportionately high defense compared to the rest of the set. Coupled with real armor for the upper body and a female character, and you have this trope.

Western Animation

The Kyoshi Warriors' uniforms from Avatar: The Last Airbender.
In the Danny Phantom episode "Beauty Mark" Sam wears one.