The S3 is the battalion's operations and training officer. A member of the battalion's command staff, an S3 typically holds the rank of major or lieutenant colonel in the Army. The S3 is also the battalion training officer, ensuring the ongoing military education of the battalion's personnel.
The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic protists. Most of them possess a unique form of organelle that comprises a type of plastid called an apicoplast, and an apical complex structure.
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.