What is an Endpoint?

It's one end of a communication channel, so often this would be represented as the URL of a server or service.

An endpoint is a URL pattern used to communicate with an API.

All of the answers posted so far are correct, an endpoint is simply one end of a communication channel. In the case of OAuth, there are three endpoints you need to be concerned with:

1. Temporary Credential Request URI (called the Request Token URL in the OAuth 1.0a community spec). This is a URI that you send a request to in order to obtain an unauthorized Request Token from the server / service provider.
2. Resource Owner Authorization URI (called the User Authorization URL in the OAuth 1.0a community spec). This is a URI that you direct the user to to authorize a Request Token obtained from the Temporary Credential Request URI.
3. Token Request URI (called the Access Token URL in the OAuth 1.0a community spec). This is a URI that you send a request to in order to exchange an authorized Request Token for an Access Token which can then be used to obtain access to a Protected Resource.

Hope that helps clear things up. Have fun learning about OAuth! Post more questions if you run into any difficulties implementing an OAuth client.

Come on guys :) We could do it simpler, by examples:

/this-is-an-endpoint
/another/endpoint
/some/other/endpoint
/login
/accounts
/cart/items

and when put under a domain, it would look like:

https://example.com/this-is-an-endpoint
https://example.com/another/endpoint
https://example.com/some/other/endpoint
https://example.com/login
https://example.com/accounts
https://example.com/cart/items

Can be either http or https, we use https in the example.

Also endpoint can be different for different HTTP methods, for example:

GET /item/{id}
PUT /item/{id}

would be two different endpoints - one for retrieving (as in "cRud" abbreviation), and the other for updating (as in "crUd")

And that's all, really that simple!