JSON is "JavaScript Object Notation". JavaScript specifies its keys must be strings or symbols.
The following quotation from MDN Docs uses the terms "key/property" to refer to what I more often hear termed as "key/value".
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Objects
In JavaScript, objects can be seen as a collection of properties. With the object literal syntax, a limited set of properties are initialized; then properties can be added and removed. Property values can be values of any type, including other objects, which enables building complex data structures. Properties are identified using key values. A key value is either a String or a Symbol value.