A conversion is:
By definition when someone makes a purchase after clicking on your ad
When someone reaches your landing page after clicking your ad or video
Any interaction with your ad that can be measured, like watching a video for a certain length of time
An action defined as valuable to your business that someone takes after clicking on your ad
An action that’s counted when someone interacts with your ad (for example, clicks a text ad or views a video ad) and then takes an action that you’ve defined as valuable to your business, such as an online purchase or a call to your business from a mobile phone.
Read more here: https://support.google.com/partners/answer/6365