It seems like there might be a small typo in your request. The word you are referring to is likely "milline," but it seems it should be "milline" as a noun used in advertising. However, this term is not commonly used in everyday English, and it may not appear in all dictionaries.
Here’s a simple explanation based on the definition you provided:
Milline (noun): An advertising measure that refers to one agate line (a small line of text) appearing in one million copies of a publication (like a newspaper or magazine).
In more advanced discussions, you might encounter "milline" when analyzing the cost-effectiveness of advertising strategies, particularly in print media.
"Milline" is a specialized term used in advertising to measure how one line of text appears in one million publications. It’s best suited for professionals in marketing or media.