"The" and "A" at the beginning of explanation

I am little bit confused about the usage of the article "The" and "A". For example, following pages start their description from "A".

A laptop (also laptop computer), often called a notebook, is ...

A dictionary is a listing of words in one or more specific languages ...

On the other hand, following pages start with "The".

The electron is a subatomic particle, symbol ...

The pipe organ is a musical instrument that ...

Each word is countable noun and they do not have any context because all of them are first line of the explanation.

Even the purpose looks same for me. I mean, all of them explain abstract nature of the noun.

What is the difference "The" and "A" in such the case ?

asked May 28, 2020 at 15:24

Gre-san

1191 bronze badge

This is a great question. As a native speaker, I find that the sentences you've quoted become less idiomatic if you replace "A" with "The" or vice versa, but I cannot explain why.
– phoog
May 28, 2020 at 15:32
A/an noun = an example (from among many) of a noun. "A dog is a good pet." ++ The is a demonstrative adjective related to that/those. The noun = the noun that we know or is described. e.g. "The cat on the table" - on the table describes the cat. "The noun" = the noun with which everyone is familiar, "The moon is bright." ++ If you approach a stranger and say "The dog is big." He will think you are mad, and say, "Which dog?" If you say " "A dog is a good pet.", he will probably agree.
– Greybeard
May 28, 2020 at 15:56
2

Does this answer your question? Is there any difference in meaning between the definite, the indefinite, and the zero article when we make generic nouns? Choice may be influenced by how prestigious the referent is deemed to be (the tiger is a majestic animal, the ...' / 'an ant is a social insect ...'.
– Edwin Ashworth
May 28, 2020 at 16:04
"The" is used when referring to a quintessential/ideal entity, in addition to it's use for a specific entity.
– Hot Licks
May 28, 2020 at 16:17

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"The" and "A" at the beginning of explanation [duplicate]