Race

In the Tensei Shitara Slime Datta Ken universe, all races are classified into two basic categories, physical and spiritual beings, each of which are further divided into three more sections: Humans, Demi-Humans and Monsters for physical existences, and Angels, Spirits and Demons for spiritual existences.

Terminologies
It is to be noted that all beings, regardless of physicality or spirituality, with some form of "evil" alignment are referred to as "Demonic-kin" 「魔族, Mazoku」. This can be especially confusing in translations as Mazoku is written as both 'demons' and 'monsters' in different circumstances. Mazoku includes all of Monsters, Demons and Devils (魔 meaning "evil" or "demonic").

Next are Monsters 「魔物, Mamono/Mabutsu meaning "Demonic Beings/Creatures"」. They include all physical beings that don't have a 'proper civilization' and don't have many, if not any, relations/treaties with civilized races. Examples are Goblins, Orcs, Fanged Wolves, Slimes, Black Serpent, Giant Spider, Lizardmen and many others.

Devils 「魔人, Majin meaning "Demonic People"」 refer to any humanoid Mazoku. This includes Beastmen, Ogres, Kijin, Harpies, Dragonoids and individuals like Gelmudo, Clayman and Laplace. Sometimes, depending on their reputation and power, certain Demi-Humans may also be referred to and a Devil. Also, Demons that take on a human or humanoid appearance when summoned can also be called a Devil.

Finally Demons 「悪魔 Akuma」 are very specific. They only refer to the spiritual beings known as Demons. They are a race of their own and have their own evolution chain.