[jon_hendry]

"it's related to the word "popular.""

Well, no, it isn't. Many things are notable that are not "popular", or even common. Leprosy, flesh-eating bacteria, progeria, Jeffrey Dahmer.

There's also a vicious cycle here: the existence of an article on Wikipedia may lead people to not post related articles elsewhere on the web, instead relying on the WP article. That may lead in turn to the article's subject being considered insufficiently notable.

[raganwald]

You measure it by counting how often people write about it in certain contexts. That seem to be "related" to popularity to me, but if you don't think so, that's fine, go your own way with my blessing.

[jon_hendry]

To me, popularity implies being seen in a favorable light, whereas "notable" is more like the absolute value of popularity or un-popularity.