Before understanding the difference between the two holy names Vishnu and Narayana, one must understand the meaning of these names. Swami Desikan has extensively taught various meanings of the word Narayana in the Tirumantradhikaram. Let us look at the two main interpretations that can be derived from it.
First is the phrase “nArANAM ayanaM nArAH ayanaM yasya saH”.
This means that Perumal is the abode for the Naras (the collection of imperishable or eternal entities). Similarly, He keeps those collections of eternal entities as His abode (He resides within them). To explain it more simply, ‘ram’ means that which is perishable, and ‘nara’ means that which is imperishable. ‘nArangaL’ refers to the collection of imperishable entities. Narayana is the one who becomes the abode for these collections of imperishable entities. That is “nArANAM ayanam”.
He also keeps these collections of imperishable entities (nArAH) as His abode. That is “nArAH ayanaM yasya saH”. Thus, we derive from the word ‘Narayana’ that the Lord becomes the support or foundation (Adhara) for everything.
When we look at the word Vishnu, it is said “caracara bhUtESu vESaNAt viSNu RcyatE”. It can be said that the Lord pervades all these movable and immovable things; that is, the meaning is that the Lord pervades everything.
When looking at both, we get a common meaning that the Lord exists everywhere. However, Nammazhwar establishes in the seventh decad of the second centum of Thiruvaimozhi (starting with ‘kEsavan tamar’) that both names indicate different qualities. He recites the twelve names (Dwadasha Nama) of the Lord. Each pasuram in the Thiruvaimozhi denotes a specific quality. Swami Desikan compiles all those qualities in the Dramidopanishad Tatparya Ratnavali. He states that each pasuram sings of a specific quality at the end of every decad. When speaking in this manner, the verse that mentions the name Narayana starts with “nAraNan mUvEzhulagukkum nAdan vEda mayam”, he says that the quality described is ‘sarva nAthaH’ (Lord of All). The Lord is the Master of everyone because He is within everything and keeps everything as His abode.
Similarly, the pasuram associated with the holy name Vishnu is ‘viTTilangu senjodi’. The quality shown there is ‘vishvak vyAptAdi vittaH’. It means that the Lord pervades and illuminates this entire universe. In particular, it is stated in the commentaries that He takes devotees as His abode and pervades within them. Therefore, this holy name is distinguished.