An action that's counted when someone interacts with your ad (e.g. 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.
Conversions are measured with conversion tracking. You can use different tracking processes to measure conversions depending on the action someone takes when they interact with your ad. Conversions can be tracked across different surfaces (e.g. mobile, desktop) and may include modelled conversions. Modelled conversions use aggregated and anonymised data to estimate conversions that Google is unable to observe directly. This can offer a more complete report of your conversions.