Is a brushless motor DC or AC? Interesting question. Well, first of all what is AC. Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously. So, an socillating signal is not always AC. It could be oscillating in value but the current flow is always in one direction, positive or negative. In order to be called AC it must cahnge pahse, so it must cross the 0 value and alternate between positive and negative. 

In the picture on the side, we can see 4 types of signals. Some people will call the blue line AC just because it is oscillating, but the blue line is never below the 0 value, so is not alternating. So, lets think for a moment. Where is this AC signal in brushless motors?

We use a battery to supply these motors and batteries are DC. The signals we apply to the coils are just pulses of DC voltage. On each step of the ESC sequence, we apply positive to one coil and GND to another coil. There is no AC. Yes, I know that you might have seen some representations of the signals which look like AC because they cross the 0 value, but those are actually including the back EMF from the motor which is always reversed, a DC motor would also do that. 

So mathematically speaking, brushless motors would act as a basic DC motor but the commutation is made electronically instead of using the shaft connectors and mechanically make the commutations.   

Imagine this: with a normal brushed DC motor, we use copper or carbon brushes to apply voltage on the coils. On comon DC motors we have 3 copper pads inside and these brushes will touch the pads while rotating, creating different polarity magnetic fields and rotating the rotor. With brushless motors, we create the same pulses of DC voltage but instead of using those pads and the brushess, we use the ESC and electroncially make the commutations. 

So, is a good question. So one hint may be the fact that brushless motors are permanent magnet motors (PM) that kinda rules out AC operation, also the term "brushless" refers to a motor that while it has no physical brushes, it still runs as a commutated motor (another hint AC motors don't have Kv ratings, they are frequency dependant), but in this case with a brushless DC controller providing the commutation instead of mechanical brushes and commutator. AC motors, such as the most familiar induction type motors are not "brushless" motors at all, they were developed about 80 years before "brushless" motors appeared in the 1970's. You will find that in every industry that manufacters and uses "brushless" motors, aerospace, computer, you name it, they are universally classed as Brushless DC motors.