Software reset
From Sega Retro
Most software produced for the Sega Saturn and Sega Dreamcast have a sofware reset feature. As the name suggests, this involves "resetting" the "software", returning the user to an earlier state, though is implemented by software recognising button combinations, rather than through a physical switch on the hardware.
Saturn
The software reset was mandated by Sega for the Saturn[1] and is performed by pressing 
+
+
+ START simultaneously. Saturn games should be able to respond to the software reset at any point (save for when memory operations are occurring[1]) and return the player back to the title screen[1]. If the player is already on the title screen, the software reset should send them back to the BIOS (or "audio CD control screen" as it was also called)[1].
Any configured options must be retained during a software reset[1].
Dreamcast
The software reset was also mandated by Sega for the Dreamcast except for "cases of special processing" (memory operations)[2]. Here the command is ++
+
+ START through regular controllers, however variations exist for other controllers[2]. Like the Saturn, the soft reset should return the user to the title screen, or if he/she is already at the title screen, to the BIOS[3].
For the Dreamcast Keyboard, the software reset is performed with Ctrl + Alt + Delete [4]. The Dreamcast Gun requires ++ START [3].
The software reset is especially important with the Dreamcast as a physical reset button does not exist on the unit. This means the hardware can only be reset by turning the power on and off.