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, but the term property is preferred in the context of web APIs a resource objects. The term property defines resource object relationships better, which is evident with nested properties and sub-properties. Therefore we would observe that the above resource object has an address property.