Defined in program.ts. The compilation context (a concept we covered previously) is represented within the TypeScript compiler as a Program. It consists of SourceFiles and compiler options.
Its interaction mechanism with the OE:
Program -uses-> CompilerHost -uses-> System
The reason for having a CompilerHost as a point of indirection is that it allows its interface to be more finely tuned for Program needs and not bother with OE needs (e.g. the Program doesn't care about fileExists a function provided by System).
There are other users of System as well (e.g. tests).
The program provides an API to get the Source Files getSourceFiles(): SourceFile[];. Each is represented as a root-level node for an AST (called SourceFile).