What metrics do you use to define product adoption?

It depends what is meant by "product", who is the intended target audience, and how product utilization drives meaningful business metrics like revenue growth, NRR, and LTV.

For products already available to customers, it really is about activation (i.e. getting customers to turn a feature on) and utilization (i.e. sustained usage by a type of user over some window of time). Utilization is typically measured daily, weekly, or monthly across individual active users or at the account level.

Last point I'll make here is all of these variables should be determined based on the synthesis of data across product usage and revenue. For example, we define adoption of Brex's accounts payable product based on a minimum viable number of invoices (say 5) paid over 3 consecutive statement periods to filter out customers simply trialing the product.

That's because the underlying adoption initiative is focused on maximizing net retention (NRR) through product attach, which we've found is achieved when customers use this product for 3 consecutive statement periods.