The correct spelling is Ummah or Umma. The word literally means community or people. When it comes to Islam, it means the whole community of Muslims bound together by ties of religion- OED. You are right that there are many sects within the Muslim community, but the whole of it is considered Ummah. The word encompasses everyone who is a Muslim.
The Arabic word /ummah/, according to Mufradat by Raqib, means any group or nation that a common cause joins them to each other, such as being common in religion and creed, being in the same time or Age, and being in one definite place, whether this unity is optional or non-optional.
My focus here is on the translation of the word Ummah (أُمَّة) in Arabic, by which Allah attributed Ibraheem in verse (16:120). As shown below there are different interpretation in its translation:
quran - Why Ibraheem was referred to as Ummah (أُمَّة) in this verse ...
According to the accepted answer in the linked question below, an ummah is " the whole community of Muslims bound together by ties of religion". But didn't earlier Prophets have their own ummah? I...
What makes one a part of the Ummah? - Islam Stack Exchange
Why is there no longer a set leader of the Muslim Ummah? Main Leader is Mahdi S.A. and is already supporting and guiding Muslims but not publicly as all can see him (this does not mean no one can meet him).
Objection may also be raised to its content, that a common fasting person from this Ummah has superiority over Moses (عليه السلام). Such a thing is contrary to our creed that the Prophets have superiority over all creation.