an alphabetical list of words relating to a specific subject, text, or dialect.
Ad Extension is the extra space provided by Google in Ads. In this, one gets more space to share more information, benefits, services of one's products and business. It also helps make ads more attractive. It helps improve CTR, keywords quality score, and ad relevancy, and generates more ROI/clicks with a low price.
There are 11 Ad Extensions available in Adwords,
3) Callout extensions