See the specification.

A declaration consists of a property name, followed by a colon, followed by a property value (colloquially, these are often shortened to property and value).

The particular type of property value is defined on a per-property basis. For width the value must be a length, percentage, or auto. You are using a length which consists of a number followed by a unit identifier.

property value

its just the css syntax without the selector

reference: https://www.tutorialspoint.com/css/css_syntax.htm

style is a special attribute that allows css style inline
width is a property (css property)
and 100px is its value

https://www.w3schools.com/html/html_styles.asp

E.g. what is the technical term for the parts of width: 100px within the following style attribute?

<div style="width: 100px;"> ... </div>

Is ‘width’ here one of the style attribute’s properties? And then what is the technical term for the ‘100px’ bit here?