Niyyah (Arabic: نیّة) means to make a wish or intention.
Terminologycally Imam Nawbi RA said that, having a strong desire for any work is called intention or Niyyah. (Faizul Qadir, Volume I, Page No. 30)
The Necessity of Niyyah
No act of worship is accepted to Allah SWT except Niyyah. Because the Prophet (peace and blessings of Allaah be upon him) said,
إنَّمَا الأعْمَالُ بِالنِّيَّاتِ وَ إنَّمَا لِكُلِّ امْرِىءٍ مَا نَوَى
Meaning: The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended. - Sahih al-Bukhari 1
Is it necessary to do Niyyah in Arabic?
Niyyah is required for every act of worship to be pure. Niyyah is also required at the beginning of the great act of worship like Hajj or Umrah.
And the intention is to decide something in the heart. And that is only the work of the heart, so in case of any intention of worship, it is not necessary to utter Arabic words.
Yes, if one's mind is not fixed without uttering his intention, then he can utter the words of intention in his mouth.
In this case, it is enough to pronounce in one's own language, there is no need to repeat long and wide Arabic sentences - (Fathul Qadir 1/26).
Niyyah to do during Hajj and Umrah
Hajj and Umrah pilgrims will clean the fingernails, mustache, and navel and armpit hair before Ihram. Then take a bath and wear Ihram clothes. Those who perform Hajj will Niyyah to perform Hajj and those who perform Umrah will Niyyah to perform Umrah.
Hajj and Umrah will be the Niyyah of the performer for either of these two acts of worship. This Niyyah is verbally agreed with the Shari'ah. Here are the Niyyah of Umrah and Hajj: