Vishnu sahasranamam , slok51

Slok51:     Dharmakruth  Dharmakruddharmee  saDasath  kshara maksharam
                   Avignaatha  sahasraamsur  vidhaathaa  kruthalakshaNaha

The one establishes Rightuousness, the one does Rightuous acts, the  personified Rightuousness, the existing and non-existing,  Perishable and non-perishable, the unconceivable, the single entity with thousand portions of Divinity and, the Brahma with all Creative Characteristics , is Sree Maha Vishnu.

Vishnu is Invisible and Unimaginable to ordinary  beings.
But , when Rightuousness  declines and unrightuousness  dominates, Vishnu takes a form and incarnates to establish the Dharma. That form is named " Avatar". For example, Ramaavatar and Krishnaavatar in Threthayug and Dwaparayug  successively.
The invisible becomes visible, when incarnates. That is Vishnu.

Vishnu is perishable wheñ inside Living beings of earth, either movable unmovable.
Vishnu is  both Bhoothathma (jeevatma) and  Paramatma. When present in living being, 

Vishnu is called BhoothAtma. The all pervading  Vishnu is Paramatma.

Brahma is the functional name of Vishnu. While creation, Vishnu becomes Brahma.

The absolute purity is VISHNU.

Om Vishnave namaha