What is the meaning of Vishnu (विष्णुः) ?
विष्णुः ( Vishnu) is made of विष् धातु (to pervade, to spread) when used with नुक् प्रत्यय
As mentioned by Adi Sankara in Vishnu Sahasranamam commentary
“वेवेष्टि व्याप्नोतीति विष्णु:” – the one who pervade everything is called as Vishnu
व्याप्ते मे रोदसी पार्थ क्रान्तिश्चाभ्यधिका स्थिता।
क्रमणाच्चाप्यहं पार्थ विष्णुरित्यभिसंज्ञितः – Mahabharata Shantiparva 341.42
O Partha, this earth and sky resides in me and and I pervades everything, because of this pervasiveness I am called as “Vishnu”
यस्माद्विष्टमिदं सर्वं तस्य शक्त्या महात्मन: !
तस्मादेवोच्यते विष्णुर्विशेर्धातो: प्रवेशनात् – Visnhu Purana 3.1.45
The power of that great Atman is pervaded in this whole universe, that is why it is called as Vishnu, because विष् धातु means to pervade.