MUTTAQI or MUTTAQUN
It is an Arabic word used as subject or object in a sentence. It actually meansa pious and righteous person who fears Allah much (abstains from all kinds of sins and evil deeds which Allah has forbidden) and loves Allah much (performs all kinds of good deeds which Allah and his Messenger (SAW) have ordained). 
The arabic word "Muttaqun" first appears in the Qur'an Surah Baqarah 2:2. It means: Pious and righteous persons who fear Allah (Glory be to Him) much (abstain from all kinds of sins and evil deeds which Allah (Glory be to Him) has forbidden) and love Allah (Glory be to Him) much (perform all kinds of good deeds which Allah (Glory be to Him) has ordained). 
Ibn Abbas(R) also said that Al-Muttaqin means:"Those who fear Allah's punishment, which would result if they abandoned the true guidance that they recognize and know. They also hope in Allah's mercy by believing in what He revealed.'' 
This is the Book (the Qur'an), whereof there is no doubt, a guidance to those who are Al-Muttaqun(conscious of Allah). Qur’an Surah Baqrah 2:2 
The root word for Muttaqi or Muttaqoon is Wa-Qa-Ya and these comes in Quran for 258 times.
Narrated Abu Sa`id Al-Khudri(R):The Prophet (ﷺ) said, "When a funeral is ready and the men carry the deceased on their necks (shoulders), if it was pious then it will say, 'Present me quickly', and if it was not pious, then it will say, 'Woe to it (me), where are they taking it (me)?' And its voice is heard by everything except mankind and if he heard it he would fall unconscious." Sahih al-Bukhari 1316 (Vol. 2: 402) 
Abdullah b. Amr(R) reported Allah's Messenger (ﷺ) as saying: The whole world is a provision, and the best object of benefit of the world is the pious woman. Sahih Muslim 1466k (Book 8, Hadith 3465) 
Allah mentions few characters of a Muttaqi in the beginning part of the Qur’an, Surah Baqarah 2:3-4.