OAuth client
An OAuth client is software that requests access to a protected resource, either on behalf of a user or on its own behalf.
The application asking for access
If a reporting app asks to read your files, the reporting app is the OAuth client. The file API is the resource server. The authorization server issues the access token that the client presents to the API.
A client can run on a server, a phone, or a desktop. It can act for a signed-in user or, with the appropriate permissions, on its own.
A client ID is not a password
The client ID identifies an application’s registration. It is public information and cannot authenticate the client by itself. Finding one in a log tells you which registration a request names, not whether the software making that request should be trusted.