The Deflicker is a setting in Super Smash Bros. Melee and Brawl that is found under options. It adjusts the way the screen appears when the game is played. When deflicker is turned off, less blurring is applied to the edges of objects, causing a sharper appearance, at the cost of featuring more jagged lines ("jaggies"). When deflicker is instead turned on, more blurring is applied to edges, giving a slightly blurrier look to the game while also removing most of the jagged lines.
In Brawl's NTSC release, the menu option for the Deflicker is a capital letter D overlaid on several straight lines; in the PAL release, however, the D was replaced by a zero, as to reflect the various languages of the game.
The Deflicker setting was created due to a majority of the population owning standard definition television screens at the releases of Melee and Brawl, with the GameCube and Wii having a maximum output of 480p. While high definition screens were available at the time, they were often too expensive for the average consumer to purchase. The relatively low pixel count of these screens could only display so much detail per frame, which became an issue as game were becoming more graphically demanding as time progressed. This mode was added to mitigate this issue and make sure everyone can clearly decipher what is happening on screen.
The Deflicker setting did not return in Smash 4 and Ultimate due to high definition becoming industry standard at that point. The Wii U has a maximum output of 1080p. The Nintendo Switch handheld screen has a maximum output of 720p, which increases to 1080p in docked mode.