When trading, the term parity can be used in several ways, but it is always used to express equality.
The most common application of parity is in the forex market, where it denotes the point at which two currencies have the same value. That means the exchange rate between the two currencies is exactly one to one.
Parity is also used in options when the value of an option equals its intrinsic value, or on exchanges when multiple bids are identical and thus equal.