العضو في المجلس.

It can be either, depending on context.

• As a complete nominal sentence, it means The member is in the council.
• As a noun phrase, it can mean the member in the council.

Arabic often allows this kind of ambiguity when there is no extra context. In isolation, many learners are taught to read it first as a nominal sentence.

Because in Arabic, the verb to be is usually not written or spoken in the present tense when making a simple statement.

So:

• العضو في المجلس = The member is in the council

There is no separate word for is here.

But in other tenses, Arabic does use a verb:

• كان العضو في المجلس = The member was in the council

This is one of the most important differences between English and Arabic sentence structure.

في is the preposition in.

So:

• في المجلس = in the council / in the assembly

Grammatically, في causes the following noun to be in the genitive case, so المجلس becomes المجلسِ in fully vocalized Arabic.

Depending on context, English might translate the whole idea more naturally in different ways, but the basic meaning of في is still in.

These are different structures with different meanings.

• عضو المجلس = a member of the council / council member
• العضو في المجلس = the member in the council or the member is in the council

So:

• عضو المجلس is an iḍāfa structure, showing belonging or relationship.
• في المجلس is a prepositional phrase, showing location or presence.

This is a very important distinction. If you want to say someone belongs to the council, عضو المجلس is usually the better structure. If you want to say where the member is, في المجلس works.

Because الـ is the definite article, meaning the.

• العضو = the member
• المجلس = the council

Arabic marks definiteness directly on the noun. So having الـ on both words tells you that both are definite.

If you removed الـ, the meaning would change:

• عضو = a member
• مجلس = a council / an assembly

If we read it as a sentence:

• العضو is the subject/topic (مبتدأ)
• في المجلس is the predicate (خبر)

So the structure is:

• العضو = the thing being talked about
• في المجلس = the information about it

This is a very common Arabic pattern:
noun + prepositional phrase

For example:

• الكتاب على الطاولة = The book is on the table
• العضو في المجلس = The member is in the council

A fully vocalized version is:

الْعُضْوُ فِي الْمَجْلِسِ

Here is why:

• الْعُضْوُ ends in ـُ because it is the subject/topic and is therefore nominative
• فِي is a preposition
• الْمَجْلِسِ ends in ـِ because it comes after في, so it is genitive

In normal pause, especially in less formal reading, learners often hear the final short vowels reduced or dropped, but in careful MSA grammar they are there.

It is pronounced roughly al-ʿuḍwu in careful MSA.

A few important points:

• The ع is the deep throat sound ʿayn
• The ض is an emphatic ḍ
• The final و is part of the word itself

So no, that و is not a plural ending.

The singular word is عُضْو = member.
The plural is أعضاء = members.

That is a useful thing to remember, because beginners sometimes assume final و must mean a masculine plural ending, but here it does not.

Yes. عضو can mean:

• member of a group, committee, council, etc.
• organ or body part

Context tells you which meaning is intended.

For example:

• عضو في المجلس = a member in the council
• عضو في الجسم = an organ in the body

In your sentence, because of المجلس, the meaning is clearly member.

In careful MSA, it is pronounced roughly al-majlis.

A few notes:

• The ج in standard classroom MSA is usually taught like j in judge
• The ال is pronounced clearly here, because م is a moon letter, so there is no assimilation
• So it stays al-majlis, not something like am-majlis

As a full sentence, a careful pronunciation would be:

al-ʿuḍwu fī al-majlisi