"Cross account" refers to services that let you manage multiple accounts with the same provider. For example, credit unions let you sign up for cross-account access if you have your name on more than one account. Google uses cross-account conversion to make it easier for businesses to manage multiple AdWords advertising accounts.
With a cross account, a credit union member might have an account in her own name, a joint account with her spouse and an account in her child's name. Signing up for cross-account access lets her move money between accounts easily, as long as her name is on them. She can transfer money to the joint account, for instance, if her spouse needs to withdraw cash, or place money in her child's account for college expenses. It's not an automatic service, so members have to sign up for it. The service doesn't give joint account holders the right to access the other member's individual account.
Google Cross Account
When a business uses Google's AdWords, one of the things it needs to know is the conversion rate -- whether customers who click on an online ad buy something afterwards or sign up for a newsletter. Cross-account conversion lets the company compare conversion rates on different AdWords accounts. That makes it easier to decide not only which ads work best, but which keywords generate the most conversions. It's faster than managing each account's conversions separately.