A CUID (Collision Resistant Unique Identifier) is a method of creating a unique identifier for use in web applications. The purpose of CUID is to create unique IDs to support horizontal scaling and sequential lookup performance better. CUID can be used as HyperText Markup Language (HTML), element identifiers, and server-side record lookups. Eric Elliott developed CUID, which can be generated and set by the app owner at the time of user registration. This allows app owners to follow user journeys across different devices.