In the CSS rule you provided, the selector is h1.
In CSS, a selector is used to select the HTML elements that you want to style. In this case, h1 is selecting all <h1> elements in the HTML document and applying the styles defined within the curly braces {}.
The other terms you mentioned, such as color, font-size, and px, are properties and values that are part of the CSS rule, but they are not the selector itself.