Sword Lines

Luminescent lines or gradients added by artists and animators to illustrate the speed and movement of a really fast swipe of a weapon. Only appearing occasionally in 2D games, they have become virtually ubiquitous during the 3D era. Usually comes in three primary varieties:
 * A thin line added to the very tip of the blade. Most commonly used on lightweight, slender swords such as the katana or the jian.
 * A wide encompassing arc that follows the entire blade. Often seen on heavy broadswords and BFSes.
 * A mixture of the two: Multiple lines that perpendicularly follow the blade swing.

Note that Sword Lines aren't just limited to swords. Axes, sabres, staffs, clubs and even arrows have a high tendency to leave behind conspicuous lines which portray their path. Somewhat justified with Laser Blades.

Arguably Truth in Television: Due to persistence of vision, after a sword is swung you can often see an arc.

A form of Motion Blur. Compare with Sword Sparks. See also Razor Wind, in which the lines are turned into a deadly ranged attack. For guns that have this effect, see Every Bullet Is a Tracer. For when only the sword lines are shown in a possible Gory Discretion Shot, see The Hit Flash.

Anime and manga

 * Samurai Champloo pulls this off quite nicely, since its highly stylized look makes them appears like streaks of lightning.
 * Death Note does this one with pens.
 * Used in the film adaptations of Kara no Kyoukai: to illustrate the speed of the protagonist's knife attacks, but done sparingly, unlike Audible Sharpness.
 * Karas makes full use of the trail effect to maximum potential.

Comic Books

 * In Bone, the Hooded One's scythe leaves a visible arc behind it when it's swung.
 * Essentially, almost every time someone swings a blade in Western comics, you can expect this. Examples are numerous and include, but are certainly not limited to, Wolverine claw swipes or when Conan the Barbarian swings his sword in the comic versions.

Film

 * V's daggers do this in slow-motion.

Live Action TV

 * This happens a lot in Tokusatsu.

Video Games

 * The Cyborg Ninja's High Frequency Blade in the Metal Gear Solid series (and the spinoff Metal Gear Acid) tends to leave a Type-2 trail.
 * Devil May Cry. The lines are usually transparent, but visible (Force Edge specifically disrupts the air when swung). And then there's Devil Trigger.
 * Sword Lines are a graphical option in Neverwinter Nights 2.
 * Sword Lines are a constant in Ninja Gaiden.
 * The Soul Series has Type-2 trails that activate from the character's attacking weapon or limb, meaning punches, kicks, headbutts and other body attacks can leave trails too. The trail gains electric sparks during Guard Break attacks and starts burning during Unblockable attacks. Each character leaves a uniquely coloured trail, and this colour is customisable in Soulcalibur 5.
 * Same for bladed weapons in Super Smash Bros. Melee and Brawl.
 * Marth's Dancing Blade technique is a prime example of this trope, as the color of the blade's trail in Brawl is dependent on the input of the Control Stick/Directional Pad, with red being neutral/forward/side, blue being up, and green being down.
 * Samurai Shodown goes here as well. Charlotte's version with her rapier treads into Razor Wind territory, at least in the first games: the 'lines' drawn by her sword remain lethal even well past when they were 'drawn' by the tip of her sword.
 * Battle for Wesnoth uses these.
 * They turn different colors depending on damage in Rune
 * Seen early on in Dark Vengeance
 * The cipher in Strider fills a sizable chunk of the screen with a crescent of light.
 * Castlevania: Symphony of the Night. One sword's line even has the word "verboten" written on it.
 * A bizarre variant occurs in Portrait of Ruin's Sister mode. When using Stella, she hardly ever visibly swings her sword, but sword lines still appear and slice enemies. Because you draw the sword lines yourself.
 * Zero in the Mega Man X and Mega Man Zero series. His Z-Saber swings drew bright green arcs or sometimes exploded.
 * The Beam Katanas from No More Heroes. But then, even Shinobu's sword leaves trails, and it's metal.
 * It's assumed that all weapons are somehow beam-edged, so that they can block other beam-edged weapons. This includes:
 * Sometimes used for rather strange things, like kicks and punches in Oni, or snowboarding trick Combos in 1080.
 * The Legend of Zelda: Ocarina of Time and The Legend of Zelda: Majora's Mask both have Link's swords leaving a trail that persist for a second or two after it has been swung.
 * There are Gameshark codes for Ocarina that allow complete customization of the sword lines, from the color to the persistence.
 * In some games when Link is at full health, he can fire a beam from his sword that looks like a Sword Line.
 * The Wind Waker uses Sword Lines. Cel-Shaded ones, which look exactly like on the concept-art.
 * In Final Fantasy VII, Sephiroth embodies this to a T. Even when he hasn't even swung it in a certain place, LINES STILL APPEAR. Now THAT is skill. The same thing goes for his attacks in Dissidia: Final Fantasy, primarily his Octaslash.
 * He did swing there. You just didn't see it.
 * Warcraft III has very visible sword lines for heroes wielding any variety of non-magic weapon, as well as arrow lines for Night Elf archers.
 * In World of Warcraft many special attacks do this. (Revenge, for example.)
 * Especially prevalent in Dissidia Final Fantasy, where pretty much every character with a sword gets this effect one way or another. Special mention goes to the aforementioned Sephiroth, who seemingly produces multiple sword strikes from one swipe of his enormous sword.
 * Sephiroth has been training with Fate/stay night's Assassin, obviously.
 * Ezio's blades in Assassin's Creed II have the "line coming only from the tip because the blades are relatively slender" variant. There's a digitized aspect to the lines, which makes sense as all of Ezio's sequences take place in the Animus 2.0.
 * Some melee weapons in Left 4 Dead 2 do this, so that the animation is noticeable to the player while realistically fast.
 * In Maple Story, maxing out a Weapon Mastery skill changes the colour of the character's Sword Lines to red for that kind of weapon.
 * Touhou has a few examples, despite being a Bullet Hell Shmup: Konngara and Meira in the first two games would use their sword to draw lines, which dissolve into bullets a bit later. Youmu, from rather later in the series, did something kind of similar; her spellcards generally resulted in Sword Lines (which could last for quite some time...) and bullets coming from roughly the same area.
 * Just about every weapon from the God of War series has this, but the main offenders are the Blades Of Chaos/Athena/Exile, who start out as just having a small yellow-y trail a la the first kind, then get more spectacular (and more red) as they gain levels. By the final form, you're tossing around a rock concert's worth of glowy-ness. Sort of Justified, as all three kinds of blades are magic and are shown to be wreathed in magical flame-energy stuff when being used.
 * In Ragnarok Online, the Sorcerer skill Striking makes your weapon do this with red lines.
 * Moon Crystal has it with the sword weapon you can have.
 * Sengoku Basara does this all the time, making for very flashy and colorful combat. Sometimes more justified than others.
 * In Fate/stay night, Saber's sword Invisible Air has a special property that warps the air around it, making it invisible, so it can only be seen by the Sword Lines it makes. See the image above.
 * Assassin's special ability, Tsubame Gaeshi, slices three times with one slash, making a highly stylized series of sword lines.
 * In American McGee's Alice, the Vorpal Blade leaves noticeable trail whenever it moves, even if Alice as much as runs with it.
 * In Blade Of Darkness (Severance) special attacks are highlighted by a yellow trail behind the sword, and weapon-specials - with a red trail.

Webcomics

 * Most weapon attacks in Order of the Stick are depicted with these.
 * Used in this Sluggy Freelance strip. Though, in this case, they might simply be blood running off the blade.
 * Subverted in Homestuck. At first it looks like Rose's knitting needles have done this, but it's actually purple yarn.
 * Jack however is fond of sword lines, often doubling as The Hit Flash.
 * Dominic Deegan learns to use these as a focus for his second sight in order to bypass scrying wards.

Western Animation

 * Avatar: The Last Airbender, taking inspiration from Samurai Champloo, has this in the fight between Zuko and Jet, which led many an Avatard to believe that Zuko was using blue fire.
 * Appears very prominently during Optimus Prime and Megatron's duel in episode 23 of Transformers Prime. Notable, Prime and Megatron's respective Sword Lines are actually colour-coded, with the former's being blue and latter's being purple, matching the colours of their respective laser blasts as well.