"Sega Master System" is a meme name that SEGA developed later on to differentiate it from the Genesis.
It was called the "Sega System" or simply "Sega".
Originally the name "Master System" was used to differentiate between three commonly sold sets:
Sega Base System
2 controllers, Hang-On
Sega Master System
2 controllers, Light Phaser, Hang-On & Safari Hunt
Sega Sega-Scope 3D System
2 controllers, Light Phaser, Sega-Scope 3D Glasses, Missile Defense 3D, (possibly Hang-On & Safari Hunt too)
>All systems had Maze Game