What is the definition of one dice?
One dice is usually called a Di. Di is singular for dice. Dice is plural for two or more.
A fair die (fair dice) is a dice which isn't altered in any way, this means it has the same chance of landing on any of one of the faces. The opposite to this is a loaded dice or cheat dice. These dice have been altered in some way, maybe by changing the weight distribution so that one of the numbers on the die/dice are more likely to be rolled.