Use digital pins as analog - Arduino Micro I have an Arduino Micro and I want to use pin 6 and 8 as an analog pin, according to the manual this is possible: 


  The Micro has a total of 12 analog inputs, pins from A0 to A5 are
  labelled directly on the pins and the other ones that you can access
  in code using the constants from A6 trough A11 are shared respectively
  on digital pins 4, 6, 8, 9, 10, and 12.


What exactly do they mean by constants? I am aware what a constant is in the scope of programming, however I'm not sure what I should use to assign pins 6 and 8 as an analog pin? I assume I cannot just do an analogRead(6)?
Constants are variables that cannot change. They are constant.

In this case they are variables that have been set up in the board definition files (in pins_arduino.h in case you're interested) and are named just as they have said - A6 through A11.  This complements the existing A0 through A5 that are normally there anyway.

So you just use those names and it chooses the right pin for you:

int val = analogRead(A8);


The definitions of the pins are:

static const uint8_t A0 = 18;
static const uint8_t A1 = 19;
static const uint8_t A2 = 20;
static const uint8_t A3 = 21;
static const uint8_t A4 = 22;
static const uint8_t A5 = 23;
static const uint8_t A6 = 24;   // D4
static const uint8_t A7 = 25;   // D6
static const uint8_t A8 = 26;   // D8
static const uint8_t A9 = 27;   // D9
static const uint8_t A10 = 28;  // D10
static const uint8_t A11 = 29;  // D12


So no you cannot just use the number 8, you have to use A8 which points to pin number 26. The Arduino core then interprets that number as an analog pin (as opposed to the digital pin, which is 8) and performs an analog read on it.