Dictionary of Biblical Words:
This is akin to holiness (2 Cor. 7:1), and must be inward as well as outward. The power for it is the hope in Christ (1 John 3:3), for it is company in spirit with Christ that alone purifies and refines through faith of the action of the word. Whiteness is an emblem of purity; hence white robes, &c.
Strong’s Dictionary of Greek Words:
Meaning:
from 53; cleanliness (the quality), i.e. (specially) chastity