Holiness is hard to define because it is a genuine attribute of God. The only way creatures can know what holiness is and become holy is through God's revelation, by experiencing God directly through His Grace (Uncreated Energies). So, I'd say that Holiness is everything that God is. To become holy ourselves we need to follow Christ and become Christ-like in every way through The Grace of The Holy Spirit. Holiness also means that one is genuinely humble, so there is no room for pride whatsoever. If we are proud that means that we want attention for who we are, or that we believe we are something outside of the ordinary; this obviously violates the law of love, that is that love has to be devoid of any self-interest, it has to become one's actual nature as it is God's. Hope this helps somewhat!