Azure terminology

what is an application

this refers to the physical application and best described with examples as defining it in technical terms probably wouldnt really help.
eg: web app, web API, console application, native mobile application

what is an application registration

this refers to the "logical" application in the identity prodicer (IdP) - in this example this is Azure Active Directory.  It defines characteristics of the application that uses it. eg: single/multi tenant, logical resources it requires access to, name, homepage etc. It defines the "template"" of the application.

what is a service principal

this refers to an "avatar" to the application registration. the application registration defines the "configuration" of a physical application. this comes into its own when we start talking about multi tenanted applications where each user tenant may want to control certain aspects of the configuration.
the service principal creates an object that is templated from the application registration and allows the administrator to set certina properteries to tenant specific values.

what is an (azure) enterprise application

these are simply service principals "registered" with the tenant and made visible through the portal.  registration is done thorugh "admin consent" and discussed later

what is an organisation

container for all subscriptions

what is a subscription

this is an entity that can incur charges.  resources are created against it

what are licenses

these are agreements and terms stating usage commitements and policy of cloud enterprise applications between the vendor and the subscription. this may include a charge depending on the vendor policy.

what is a tenant

this is an instance of azure active directory that houses the users for which a many can exist to an organisation

what is a user

this is a entity representing a operator in the organisation.  allows an adminisrator to control the access and privilege of the operator.  licenses may be applied to users to allow access to licenced cloud offerings

what is a single tenant application

an application that only works in the home tenant

what is multi tenancy application

an application that works for multiple tenants.  this would normally be in chorus with a licence and enterprise application.  the home tenant service principal is registered with the user tenant via manual scripting, microsoft cloud app gallery, admin consent links etc.