Deterministic matching is a process in which a user's online activity is linked to their real-world identity in a precise and certain manner. This is typically done using personal information such as a name, email address, or phone number, which is used to match the user's online activity with their offline identity. Deterministic matching is often used in digital advertising to target specific individuals with relevant ads, and to measure the effectiveness of those ads. For example, an advertiser might use deterministic matching to serve an ad to a specific person based on their email address, and then track whether that person clicked on the ad and made a purchase. Deterministic matching can provide advertisers with more precise and accurate data about their target audience, but it also raises concerns about privacy and the use of personal information.