A smallest unit of content, and the basic building block of a content type. Content types are basically a collection of attributes.
Attributes will usually always have a set datatype.
Nomenclature varies widely. Many systems refer to an attributes as a property or field or (less commonly) a parameter. There is no standard.
Some CMS have built-in attributes which are present in all types. These are common:
Body (longer text, often allowing formatting
A content type is defined by what attributes exist in addition to these.