An Application programming interface (API) is a source code interface that an
operating system or library provides to support requests for services to be made
of it by computer programs. Advanced programming interface is a near synonym with
wider application that predates the current common usage. In the original term the
concept is meant to represent any well defined interface between two separate
programs. The main difference is that this older term does not inculcate a
parent-child relationship and can therefore be applied to peer-to-peer situations
more logically, e.g. internal kernel services which can present themselves as separate programs.

To create a private synonym in your own schema, you must have CREATE SYNONYMsystem privilege.

To create a private synonym in another user's schema, you must have CREATE ANY SYNONYM system privilege.

Synonym, as the name suggests, is an alternative name for an object. It can be used for simplifying names that are lenghy or complex, for e.g you can use a simple synonym for accessing objects of other users, for which you had to prefix username to the object.