This article will be about the different usages of the word Ummah in the Quran. It will not be about explaining the brotherhood of Muslims. That is a topic covered in many other articles by more qualified people.
The word أُمَّة is used 51 times in the Quran in its singular form, and its plural أُمَم is used 13 times.
There are 4 major meanings it is used in. I will discuss them all one by one.
The most common and obvious meaning of the word Ummah is a community or group of (usually) people. This is by far the most used meaning in the Quran. All other meanings simply derive from this original meaning in some way.
1a. Ummah is used to refer to a group of people in general.
Allah says in the story of Musa (AS):
وَلَمَّا وَرَدَ مَاءَ مَدْيَنَ وَجَدَ عَلَيْهِ أُمَّةً مِنَ النَّاسِ يَسْقُونَ وَوَجَدَ مِنْ دُونِهِمُ امْرَأَتَيْنِ تَذُودَانِ
And when he came to the well of Madyan, he found there a crowd of people watering [their flocks], and he found aside from them two women driving back [their flocks]. (28:23)
1b. Often the reason this group is considered a group is because of their blood relation, religion, culture, or another common quality.
An example of this is when Allah says:
تِلْكَ أُمَّةٌ قَدْ خَلَتْ لَهَا مَا كَسَبَتْ وَلَكُمْ مَا كَسَبْتُمْ وَلَا تُسْأَلُونَ عَمَّا كَانُوا يَعْمَلُونَ
That was a nation which has passed on. It will have [the consequence of] what it earned, and you will have what you have earned. And you will not be asked about what they used to do. (2:134)
Here, “nation” refers to the prophets Ibrahim, Ismail, Ishaq, and Yaqub (AS) mentioned in the previous verses who are all related by blood, religion, and time.
Another example is when Allah says:
وَلَقَدْ بَعَثْنَا فِي كُلِّ أُمَّةٍ رَسُولًا أَنِ اعْبُدُوا اللَّهَ وَاجْتَنِبُوا الطَّاغُوتَ
And We certainly sent into every nation a messenger, [saying], “Worship Allah and avoid Taghut.” (16:36)
Here, it refers to tribes or generations.
Another example is when Allah says:
وَلْتَكُنْ مِنْكُمْ أُمَّةٌ يَدْعُونَ إِلَى الْخَيْرِ وَيَأْمُرُونَ بِالْمَعْرُوفِ
And let there be [arising] from you a community inviting to [all that is] good, enjoining what is right and forbidding what is wrong
Here, we know that the Muslims who do this are not all of common descent or culture. But, they are one community due to their common action.
1c. Sometimes the word is used to refer to whole species that are not even human.
وَمَا مِنْ دَابَّةٍ فِي الْأَرْضِ وَلَا طَائِرٍ يَطِيرُ بِجَنَاحَيْهِ إِلَّا أُمَمٌ أَمْثَالُكُمْ مَا فَرَّطْنَا فِي الْكِتَابِ مِنْ شَيْءٍ ثُمَّ إِلَى رَبِّهِمْ يُحْشَرُونَ
And there is no creature on [or within] the earth or bird that flies with its wings except [that they are] communities like you. We have not neglected in the Register a thing. Then unto their Lord they will be gathered.
Here, Allah refers to the species of animals as communities. So, Jinn are one community, angels are one community, and birds are one community. It can also be divided further and one can say crows are one community and sparrows are one community.
The word Ummah also refers to religion. The reason it came to refer to religion is because a community of people often follow a common religion. Thus, the word Ummah also came to refer to a religion itself.
The most obvious example of this is when Allah says:
بَلْ قَالُوا إِنَّا وَجَدْنَا آبَاءَنَا عَلَى أُمَّةٍ وَإِنَّا عَلَى آثَارِهِمْ مُهْتَدُونَ
Rather, they say, “Indeed, we found our fathers upon a religion, and we are in their footsteps [rightly] guided.” (43:22)
There are other examples that could be given, but they are more ambiguous in that they could just refer to groups like meaning 1.
For example, Allah says:
إِنَّ هَذِهِ أُمَّتُكُمْ أُمَّةً وَاحِدَةً وَأَنَا رَبُّكُمْ فَاعْبُدُونِ
Indeed this, your religion, is one religion, and I am your Lord, so worship Me. (21:92)
Someone could say this just refers to a group united by religion rather than the religion itself.
The word Ummah is used at least two times in the Quran to refer to an amount of time. It is used for time because it refers to the time it takes for a group of people to die.
وَلَئِنْ أَخَّرْنَا عَنْهُمُ الْعَذَابَ إِلَى أُمَّةٍ مَعْدُودَةٍ لَيَقُولُنَّ مَا يَحْبِسُهُ أَلَا يَوْمَ يَأْتِيهِمْ لَيْسَ مَصْرُوفًا عَنْهُمْ وَحَاقَ بِهِمْ مَا كَانُوا بِهِ يَسْتَهْزِئُونَ
And if We hold back from them the punishment for a limited time, they will surely say, “What detains it?” Unquestionably, on the Day it comes to them, it will not be averted from them, and they will be enveloped by what they used to ridicule. (11:8)
In Surah Yusuf, Allah says about the prisoner companion who forgot about Yusuf (AS)’s:
وَقَالَ الَّذِي نَجَا مِنْهُمَا وَادَّكَرَ بَعْدَ أُمَّةٍ أَنَا أُنَبِّئُكُمْ بِتَأْوِيلِهِ فَأَرْسِلُونِ
But the one who was freed and remembered after a time said, “I will inform you of its interpretation, so send me forth.” (12:45)
In this verse, it refers to something like nine years as that is the amount of time Yusuf (AS) was in prison.
According to some opinions, Ummah is also used as a synonym of Imam i.e. leader. This is due to the Allah describing Ibrahim (AS) as an Ummah.
إِنَّ إِبْرَاهِيمَ كَانَ أُمَّةً قَانِتًا لِلَّهِ حَنِيفًا وَلَمْ يَكُ مِنَ الْمُشْرِكِينَ
Indeed, Abraham was an Ummah, devoutly obedient to Allah, inclining toward truth, and he was not of those who associate others with Allah. (16:120)
Here, most people said this refers to Ibrahim (AS) being a teacher or leader that people learn from and follow. This opinion is reported from a lot of the Salaf like Ibn Masud, Qatadah, and Muqatil.
This opinion is also confirmed by the verse in Baqarah where Allah says to Ibrahim (AS):
“Indeed, I will make you an Imam (leader) for the people.” (2:124)
Others said this refers to him being the only believer of his time. So, he is considered a community by himself because he was the only community of believers.
Still others said this refers to him being worth as much as an entire Ummah of believers in terms of his effort and faith. This is similar to the previous opinion.
And Allah knows best.