Cross-device conversion refers to the process of tracking and measuring conversions that happen across multiple devices. In today's digital world, many users move between different devices, such as smartphones, tablets, and computers, when interacting with websites and apps.

Tracking cross-device conversions allows advertisers to see the full customer journey, from initial interaction with an ad on one device, to the eventual conversion on another device. This can provide valuable insights into the effectiveness of an ad campaign and allow advertisers to make adjustments as needed to optimize their performance.

For example, an advertiser might run a display ad campaign on a website, and then track the conversions that happen on that website. They can then use cross-device conversion tracking to see if users who interacted with the ad on their desktop computer later converted on their smartphone. This can help the advertiser understand the full impact of their ad campaign and make adjustments as needed.

When it comes to cross-device conversions, it's important to choose an attribution model that accurately reflects the customer journey. For example, if a customer interacts with an ad on their phone, but doesn't make a purchase until they get home and use their laptop, a last-click attribution model may not accurately credit the ad with the conversion. In this case, a linear, time decay or multi-touch attribution model may be a better fit.