Definite article in definitions

Question

I have noticed a pattern of using the definite article „the” that occurs in definitions, and I don’t know how to explain that. The pattern is as follows: "A/An + term + basic description. The + term + further details". Let’s take a few examples in which the pattern is present.

An operating system exploits the hardware resources of one or more processors to provide a set of services to system users. The OS also manages secondary memory and I/O (input/output) devices on behalf of its users.

Source: Operating Systems: Internals and Design Principles by William Stallings

A computer program is usually written by a computer programmer in a programming language. From the program in its human-readable form of source code, a compiler or assembler can derive machine code—a form consisting of instructions that the computer can directly execute. Alternatively, a computer program may be executed with the aid of an interpreter.

Source: Wikipedia's definition of a computer program

A Web application is a collection of servlets, HTML pages, classes, and other resources that make up a complete application on a Web server. The Web application can be bundled and run on multiple containers from multiple vendors.

Source: Java™ Servlet Specification Version 2.4

My questions are:

1. What is the purpose of using the definite article "the" with the noun phrase in the second sentence in each example?
2. If the purpose is to refer to the object that has been introduced in the first sentence, why don't we use "it" instead of repeating the whole noun phrase.
3. Why don't we just use an idefinite article, if the sentence in each case is true for all instances of the term? (for example, "A Web application can be bundled and run on multiple containers from multiple vendors" - any Web application can be bundled and run on multiple containers from multipme vendors).

Edit. How does it differ from the following definition of a telephone:

A telephone (derived from the Greek: τῆλε, tēle, "far" and φωνή, phōnē, "voice", together meaning "distant voice"), or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user.

Source: Wikipedia's definition of a telephone

Here we have "a telephone" in the beginning of the first and second sentences.

Answer 1

This is absolute standard use of definite and indefinite articles.

When a subject is first introduced, we use the indefinite article, because either there isn't a specific one in mind, or the speaker does not expect the hearer to be able to identify the subject. "A man came into the room". "I saw an aeroplane coming in to land". "An OS is a system ..."

Once this indefinite subject has been introduced, it becomes definite, identified, for the purpose of the discourse, and is referred to with a definite article. "The man was holding a camera in his hand". "The aeroplane banked sharply and headed for the airfield". "The OS has the following properties". If you used another indefinite article, it would suggest that you were talking about another element, not the one already introduced.

As for your question 2: we can refer to element by an anaphoric pronoun ('he', 'it' etc). Whether we do so or not is a matter of style and clarity.

[Note about narrative: Occasionally in narrative, a writer will break that first convention, and refer to something new in the story by a definite article. The effect of this is to create the impression that the narrator, and possibly the reader, already knows the element. So, starting a story "The man came into the room" creates a back-story, suggesting that the narrator already knows who the man is, and (in the fiction) expects the reader to know this too. ]