In plain language
It combines “find, then update or create” into one step, so the workflow does not create duplicates.
Example
Each new form submission upserts a contact by email, updating the existing contact or creating it if the email is new.
Related terms
Deduplication · Record · Race Condition · Sync (One-Way and Two-Way)
Common misconception
An upsert is only as reliable as its key; matching on a field that is not unique, such as a name, still merges or duplicates records.
Implementation guidance
Upsert on a truly unique key, normalize that key first (for example, lowercase emails), decide which fields an update may overwrite, and use native upsert where the API offers it.