Wiktionary
multirotor
a. Having or relating to more than one rotor.
Wikipedia
Multirotor
Due to their ease of both construction and control, multirotor aircraft are frequently used in radio control aircraft and UAV projects in which the names tricopter, quadcopter, hexacopter and octocopter are frequently used to refer to 3-, 4-, 6- and 8-rotor helicopters, respectively.
In order to allow more power and stability at reduced weight coaxial rotors can be employed, in which each arm has two motors, running in opposite directions (one facing up and one facing down).