Definition of Om
OM has three Hindi words. Aa,Uu and Ma. Aa is meant for Aakaar from which three names of God are built, Viraat, Agni and Vishwadi. From Uu= uukaar from which Hirnyagarbh, Vaayu and tejas names occurs and Ma = makaar from which Ishwar, Aditya and Praajyan aadi, holy names of God occurs. Rig ved mantra 1/164/ 46 says the God is one but His names are several. But here only the said names are given. Viraat means he who creates the universe. Therefore His name is Viraat.
Agni means he who is knowledgeable, omniscient, to be known and adorable. Therefore His name is Agni. Vishw means in whose the space, fire etc., i.e., all bhoots are entering or he who is in all these bhootas and is omnipresent. So His name is Vishw. Hirnyagarbh means in whose these sun, moon i.e., dyulok reside and originated i.e., why His name is Hirnyagarbh. Vaayu means by which the universe is held , gives lives and destroy and is the supreme power, that is why His name is Vaayu. Tej means who is a shape of light Himself, i.e., God does not take light from anywhere else. But gives light to sun, moon, stars etc., So the unbroken, unconstrcted/unproduced and eternal shape of light is Almighty God and not sun, moon, stars, candle or any lamp of the world or any light of the world. Because all these lights are broken one day, but not God. Ishwar means who and whose knowledge is always a never dying truth and who has unlimited pleasure or merriment in Him. Aditya means who can not be broken in to pieces and thus cannot be destroyed. So His name is Aditya. Praajyan means He who knows the every manner and behave of whole of the universe without misunderstandings. So His name is Praajyan.