In biochemistry, p50 represents the partial pressure of a gas required to achieve 50% saturation of a particular protein's binding sites.
The term is analogous to the Michaelis–Menten constant (KM), which identifies the concentration of substrate required for an enzyme to achieve 50% of its maximum reaction velocity.
Imagine myoglobin, a protein which is able to bind a single molecule of oxygen, as per the reversible reaction below, whose equilibrium constant K (which is also a dissociation constant, since it describes a reversible association-dissociation event) is equal to the product of the concentrations (at equilibrium) of free myoglobin and free oxygen, divided by the concentration of myoglobin-oxygen complex.
From defining the p50 as the partial pressure at which the fractional saturation is 50%, we can deduce that it is in fact equal to the dissociation constant K. For example, myoglobin's p50 for O2 is 130 pascals while the P50 for adult hemoglobin is 3.5 kPa.
Thus, when O2 partial pressure is low, hemoglobin-bound O2 is more readily transferred to myoglobin.