Acne vulgaris is the medical name for common acne -- the presence of blackheads, whiteheads, and other types of pimples on the skin. The most common spots for breakouts are the face, chest, shoulders, and back.
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An application programming interface (API) is (as the name suggests) an interface for your application to use code that does not belong to your application (usually a library). Usually, an API is comprised of a set of function/method calls that can be made to the library by your application. All system calls together constitute an API that the operating system (kernel) provides to applications that run on that platform.