XML/HTML are centered around elements. An element is something that has an opening tag and a closing tag, or simply has a single self-closing tag (The slash before the closing angle bracket is required in XML and XHTML but not HTML). For example, this has six elements:
<meta charset="utf-8" />
In this example each tag name is only used once. The elements are (by order of first appearance) html, head, meta, title, body, p. The DOCTYPE definition is special and is not considered to be an HTML or XML element. The charset part of this document is an attribute.
What does element mean in JSON? Something a bit different. An XML element is a compound data type. A JSON element is part of a compound data type.
If you look at the white box on the right part of json.org detailing the grammar, you can see that it has names for items in JSON’s compound data types, objects and arrays. An item in an object is called a pair, and an item in an array is called an element.
It helps when building APIs for processing data formats to have names for things. The terminology for XML is used in XML tools, and has made them easier to understand. For instance, the jQuery API docs use the terms element and attribute heavily.
JSON has been popular for years now, in many vital technology communities, but it is still lacking in processing support compared to XML. I think clear terminology is one thing that XML has right, and indeed the JSON spec has clear terminology. What’s lacking is widespread use of the terminology.