How many times do I have to kill a player before I am dominating them?

Question

I've observed that sometimes I just have to make two kills in order to dominate a certain player, sometimes thrice or is there any other factor that matters?

This will be helpful to unlock certain achievements that depend on dominations.

Answer 1

You have to get any combination of four kills or assists on an opponent in a row (that is, without them getting either a kill or an assist on you) to dominate an opponent.

Once dominated, either a single kill or assist is enough to get revenge.

Check the TF2 wiki for more info on dominations.