Ummah is an Arabic word meaning "community". It is a synonym for ummat al-Islām أمة الإسلام, 'the Islamic community'.
Allah Subhanahu Wa Ta'ala says, “And hold firmly to the rope of Allah all together and do not become divided. And remember the favour of Allah upon you - when you were enemies and He brought your hearts together and you became, by His favour, brothers.” [3:103]
“The parable of the believers in their affection, mercy, and compassion for each other is that of a body. When any limb aches, the whole body reacts with sleeplessness and fever.” [al-Bukhari]
Muslim Ummah is now divided on so many issue, Our Prophet (saw) said, “Verily, the believers are like a structure, each part strengthening the other,” and the Prophet clasped his fingers together. [al-Bukhari]