It is usual to consider that there is an equal probability for a coin to land either heads or tails. Such a coin is called a fair coin.
However, discs of metal can crafted to look like coins, but with a higher probability of landing on one side rather than the other.
Such a device is known as a biased coin.
It is worth mentioning that as the manufacture of coins is such that the design on either side is embossed in the metal itself, their physical nature is such that they are by definition asymmetric.
Hence by their very nature all coins are to a certain extent biased to a more-or-less minute degree.
Skilled tossers are rumoured to be able to take advantage of this fact.