an alphabetical list of words relating to a specific subject, text, or dialect.
The procedure that occurs with each Google search to choose which ads will show up for that particular search and in which arrangement those ads will appear on the page (Or whether or not any ads will show at all).
Each time an AdWords ad is qualified to show up for a search, it goes through the ad auction. The auction decides if the ad really appears and in which ad position it will appear on the page.
How the Auction Works:
When somebody searches with a keyword or a set of keywords, the AdWords system discovers all ads whose keywords match with that search.
From those ads, the system overlooks any that aren't qualified, like ads which are disapproved due to policy violation or ads that are targeting a different country.
Of the rest of the ads, just those with an adequately high Ad Rank may appear. Ad Rank is a blend of your bid, ad quality,the Ad Rank threshold, the setting of the individual's search, and the expected effect of extensions and other ad formats.
Since the auction process is a repeated process that happens for each search on Google; each auction can have different outcomes depending upon the competition at that moment. So, it is obvious that your ad position will vary each time. Consequently, it's typical for you to see some variation in your ad's position on the page and whether or not your ad shows at all.