Quick Answer
In the context of a resource object, a property is a synonym, or shorthand expression, for a JSON name/value pair.
Digging Deeper
Properties are used to enumerate one or more identifying pieces of information related to the same resource object. Consider the following resource object:
In the above example, we see an address name/value pair. It is perfectly acceptable to use "name/value pair" since this is how JSON is documented. However, the term property is preferred in the context of web APIs and resource objects because it defines resource object relationships better, which is evident with nested properties and sub-properties. Therefore we would normally observe that the above resource object has an address property.