A/The liver is a major internal organ.

(2) is better.

When making what seems to be a definition, we commonly use 'the'.

Hmmm, I am not sure what is the rationale behind choosing 2...

Clive, please have a look at Wikipedia's articles. For example:

https://en.wikipedia.org/wiki/Triangle

A triangle is a polygon with three edges and three vertices.

As long as it is not a proper name with 't

But look here for the use of 'the liver' by Wikipedia.

https://en.wikipedia.org/wiki/Liver

I don't have a quick explanation.

Possibly it is related to the fact that there are many kinds of triangles whil

ClivePossibly it is related to the fact that there are many kinds of triangles while the liver is more unique.

I agree; I was about to say much the same.

The liver is a major internal organ (of the body.)

If you have a body and you dissect it, you will find its liver. We don't say "a liver" because it is a specific part of a body. There are no independent livers, as there are for triangles.

The nave is the central part of a church.

AlpheccaStarsThe liver is a major internal organ (of the body.)

The thing is, there are many rules about articles in English and at first sight they are quite contradictory. I think that the best example is speaking about things in general - we can use the definite article, indefinite article and zero article.

a) Liver is a major internal organ.
b

a) Liver is a major internal organ. No. This sounds like you are talking abut the substance called 'liver'. Having no article would only make sense if you said

eg Liver tastes good if you eat it with fried onions.

Aaa, like chicken:)

a) Liver is a good source of iron and other essential minerals. (Ø article. Like meat, bread, and pizza, liver is uncountable when used as a food.)
b) A liver is a major internal organ (of a body.) (The indefinite article is not used.)
c) The liver is a major internal organ (of the body.) (Definite article. There is one organ called "the liver" in the body of an animal. The li