Kevel documentation has a wonderful glossary of several terms used in the AdTech industry (source):


ClicksNumber of clicks on the ad. These clicks can go anywhere - a lead generation form, a website, a product page, an eBook, etc
Conversions (Actions)Number of post-click actions (such as making a purchase, filling out a form, etc)
ImpressionsWhen an ad is displayed on the site/app
Viewable ImpressionsTechnical term for number of impressions where at least half of the ad was shown for at least 2 seconds
Video ViewsHow many times a video ad was viewed. Sometimes broken down by how far the user got into it, like 25%, 50%, 75%, and 100% completed.
Cost-per-action (CPA) or cost-per-conversionTotal cost (ad spend) divided by actions/conversions ”
Cost per click (CPC)Total cost (ad spend) divided by clicks
Cost-per-mille (CPM)Total cost (ad spend) divided by thousand impressions. 1M impressions at 1.00
Click-through-rate (CTR)Clicks divided by impressions. AKA, the rate that people click on the ad after seeing it
Conversion Rate (CR)Conversions divided by clicks. AKA, the rate that people undergo a desired action after clicking

Types of Custom Ads

Companion AdsAds from the same advertiser that appear in tandem. Often refers to display ads that appear alongside a video ad, such as on YouTube
Dynamic AdsCatch-all term for ads that change depending on who is viewing it. For instance, showing rain gear to people who are currently living somewhere where it is raining
Full-Screen TakeoverAn ad that takes over the entire screen (most relevant on mobile)
In-Feed/In-Stream AdsAn ad that appears within the flow of content, such as Facebook’s mobile in-feed ad unit
Promoted/Sponsored PostNative ads - often on social networking sites - that look and feel like organic posts. Think Twitter, Pinterest, Facebook, etc
Recommendation WidgetsThe rectangular ‘around the web’ content ads you often see on media/news sites
Responsive AdsAds that automatically change size based on screen/browser size
Session TakeoverWhere you show the same user the same ad (or ads from the same advertiser) for their entire browsing session
Sponsored ArticlesArticles on news/media sites that are sponsored by a brand. Done correctly, these are not self-promotional (like H&R Block sponsoring an Onion article on taxes). However, there are also ‘advertorials’ - ads that promote a product while trying to look like objective editorial pieces
Sponsored/Promoted ListingsUsed by eCommerce, marketplaces, and any company that shows “listings”. Here, companies/people pay for their organic listing to be artificially high up in the search results. For instance, Amazon lets vendors pay to have their product be the first listing for a given search word
SponsoredA custom native ad unit sponsored by a brand. Can be anything. Tinder has “sponsored profiles”; AllRecipes as “sponsored ingredients”; it’s really up to your imagination!
Viewable AdsTo counter concerns that some ads are displayed but never seen, some advertisers may ask for ‘viewable ads’. The IAB considers a viewable ad to have 50%+ of its pixels visible for at least 2 seconds