In the context of property transactions, the best price that might reasonably be expected for property, or an interest in property, from an unconnected third party on a certain date. A valuer would normally base such a valuation on an average of values for similar properties or interests and would not take into account an unusual transaction.
The term may have a particular meaning, depending on context. For example, many leases will specify exactly what is meant by open market value (or open market rent) in the context of a rent review.