Is arduino a single board computer? First excuse me if tags are not reflecting the content,

While watching a video about single board computer (SBC) I got confused that it doesn't mention arduino as SBC. I always considered it as SBC.

After digging around a bit I got more confused. It seems single board computer is where processor, memory, I/O this things are built on single circuit board rather than using expansion card attached on to a circuit board (motherboard).
According to that is not arduino a SBC? It's processor, memory,storage are on single ATMega32A (for uno) attached on a single board which provides some I/O port (I/O pin, USB, UART, I2C etc.).

From some digging I learned that arduino code runs on bare metal unlike SBC like Rpi and that's the reason for it not being SBC. This seems weird reason. 

(1) Is Arduino a SBC?
If (1) is no then next questions are,

(2) Is Being less powerful is the cause? If yes where do you draw the line?

(3) Not having an OS hosted environment for execution is the cause? 

If (3) is Yes then,

(4) Arduino has some bootloader code to load the program through USB. Isn't it  kinda like OS, I admit it's not giving some nice api for h/w usage but is it all?

This link comment section kind of started a debate but ended without giving clear answer about my question.
The Arduino isn't a computer. It is a programmable microprocessor / microcontroller. 

Generally a computer can run multiple applications where most programmed devices like the Arduino are running only one program. This is not an official definition but it is one way to differentiate. 

It can be difficult to draw a line where it is one but not the other.


  (4) Arduino has some bootloader code to load the program through USB.
  Isn't it kinda like OS, I admit it's not giving some nice api for h/w
  usage but is it all?


No that isn't like an operating system. The Arduino does not have a real operating system when compared to a computer system. And someone being able to load in a new program is just changing the current single program that is runnable. 


  An operating system (OS) is system software that manages computer
  hardware and software resources and provides common services for
  computer programs.


I think the programs being plural is an important factor, but this is only my opinion. 

There is no one definition so you will get a number of different opinions. And most will be reasonable.