The client asks if this particular combination of data (provided as GET query parameters) is found in the database overseen by that web service. The server looks in its database (e.g. maybe it's the collection of all voter registration records for a particular country) and if there is a matching record it replies saying what was found and where.

You can run that same request again and get the same exact answer and running it many times or not at all changes nothing‡ so that seems to meet your requirement entirely.

‡ In practice some of the services do accounting, they are incrementing a counter somewhere for every query run and then we use that counter to determine the payment to a third party for the use of their data. But this is no greater deviation from the concept than the usual practice of logging GET requests and anyway most of the services don't do that.