Glossary: Z is for…

Welcome to our glossary of online advertising terms!

Find out more about online advertising terms that start with the letter Z.



The Z-Index is the order in which items on a webpage are shown if they were placed on top of each other. It helps to think of a webpage as a stack of papers, with the paper on the top level having the highest number. This is important in online advertising as if an ad has a low z-index then it will appear behind other content on the page. Generally the highest Z-Index possible is 999,999,999.

This concept is best illustrated – in the image below three circles are put on a webpage with the corresponding Z-indexes (the highest number appears “in front” of the others).

The circles have the labelled z-indexes so the blue one appears "in front" of the other two.

The circles have the labelled z-indexes so the blue one appears “in front” of the other two.


Zombie Cookie

A Zombie Cookie is a cookie which won’t die. Most cookies are automatically deleted by browsers after a period, or at the very least can be deleted by a user whenever they want. A Zombie Cookie has its data stored outside of a user’s computer however, and is reinstated on the users computer once deleted when the users computer comes in contact with where it was stored. Although not universally illegal, zombie cookies are very dodgy, and bad practise (Black Hat).


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