Flinch

Flinching, also called Hitstun is what happens to a character when they are hit by an attack. The character will be stunned for a very short period of time, usually just enough to interrupt any attacks or movements. There are certain attacks that do not cause flinching, and ways for a character to avoid flinching. The amount of flinching is referred to as hitstun. Some attacks cause flinching without any knockback whatsoever. It is used for combos in all 3 smash games. Some powerfull moves such as Knee Smash, Samus's Charge Shot (fully charged), and Zelda's Lightning Kicks have a very long and noticeable hitstun, and using a Timer can amplify the affect, allowing the possibility of near-infinite combos.

Attacks that do not cause flinching

 * Fox's Blaster (except in SSB)
 * Mario's F.L.U.D.D. (no damage)
 * Dr. Mario's and Mario's Cape
 * Squirtle's charged Water Gun (no damage)
 * Luigi's Super Jump Punch (only in Melee)
 * Link's Gale Boomerang (on the return) (no damage)
 * Ness's PSI Magnet (when finishing the attack) (no damage)
 * Snake's thrown Grenade (not the explosion)
 * Olimar's Pikmin when latched on
 * Mr. Game & Watch's Judge number 1
 * The flame of Charizard's forward aerial
 * The rain that occurs in the "Kitty Cover" game in the stage WarioWare, Inc.
 * Glancing Blows (no damage)

Attacks that cause flinching with no knockback

 * First hit of Ganondorf's Down Smash
 * First hit of Lucario Down aerial
 * First hit of Olimar's Neutral A
 * First hit of Peach's Neutral A
 * First two hits of Sonic's Neutral A
 * First hit of Sonic's Up aerial
 * First two hits of Snake's Neutral A
 * First three hits of Snake's Neutral aerial
 * First three hits of Snake's Down aerial
 * First hit of Snake's Forward tilt
 * First hit of or Young Link's or Toon Link's Forward Smash
 * Luigi's Super Jump Punch (in Melee and Brawl, non-sweetspotted)
 * Falco's blaster.

Ways to prevent/reduce flinching
In the original Super Smash Bros., hitstun was extremely high. This meant that this game had the most extreme and arguably best combos in the series, allowing almost every character to have a 0-death combo.
 * Metal Box
 * Super Mushroom
 * Certain Stickers (only in The Subspace Emissary and Boss Battles Mode in Brawl)
 * Any moves that offer knockback resistance.
 * Air dodging (Brawl only)
 * Using an aerial attack (Brawl only and best the fastest of all)

In Melee, hitstun was lowered slightly, but wasn't lowered so much that it didn't allow for any combos, because it still allowed for many, such as the Ken combo. The majority of characters have some combos, but 0-deaths are rare (and those that exist require extremely quick fingers and lots of technical skill to utilize). The Top, High, and Middle tiers all operated mostly on combo ability.

In Brawl, hitstun was severely lowered, something that the game is often criticized for by pro Smashers. The opponent suffers very little stun after being hit, and air dodging decreases it even further. Thus, the game has hardly any real combos. Some exceptions are Peach, Sheik, Kirby, and Meta Knight, who all have fast enough moves (or a select few, in Peach's case) to combo fairly well. Notable combos include Peach's Dair to Uair tactics, Sheik's Foward Tilt Lock, Meta Knight's Uair chaining, and Kirby's Forward Throw combos. However, these can be DI'd out of with near perfect SDI, and the combos rarely exceed two or three hits.