I was asked by a potential client if I can have my software interact with Esri/ArcGIS Online.
Use case: users is logged into SomeRandomSoftwareApp and is looking at a Widget, this Widget includes an Esri asset id, the user clicks a link that passes that ID to Esri/ArcGIS Online and behind the scenes the user is logged into Esri and they see the data associated with the Esri/ArcGIS Online.
Thanks, Keith
If I understand correctly, you have two options for this: API Keys or Application Credentials.
The first one, is a permanent token generated by the owner of the data that will allow the application easy access to it. This is still in beta, and it was not ready for use the last time I check some time ago.
The second one, the owner of the data will generate credentials for your application. With this credentials you will have to request a token each time you want to access the data, all this via OAuth 2.0.
Check the docs for more details ArcGIS Services - Security