Good question as definitions can certainly vary. From a Christian perspective I think Bede Griffiths is a good source to start with.
"For Griffiths, advaita is a mystical intuition of being one with the divine reality; his experience of non-duality in his encounter with God is equivalent to the experience of the soul in its very centre, beyond images and concepts. Hindus and Buddhists may express this non-dual reality differently, but Griffiths believed that their experience of the non-duality of the divine is fundamentally the same. Christians have a lot to learn from Hinduism and Buddhism in their quest for the Absolute. At the same time, Christians also have a lot to offer to Eastern religions in terms of refinement and reinterpretation of
the advaitic experience. This involves seeing the Hindu notion of advaita in the light of the Christian understanding of creation, the notion of the person and the incarnation of Jesus Christ.
Rejecting the monism of pure advaita, which affirms the absolute identity between Brahman and the soul, Griffiths describes a Christian advaita characterized by intuitive knowledge, love and an affirmation of the reality of the world. He believed that
individuals do not lose their identity, even in deep communion with God. Relationship with God does not abolish the individuality of the soul. The relationship cannot be one of total identity or complete absorption.
Griffiths writes:
For the Hindu and the Buddhist … in the ultimate state there is an absolute identity. Man realizes his identity with the absolute and realizes that this identity is eternal and unchangeable.
In the Christian view man remains distinct from God. He is a creature of God, and his being raised to a participation in the divine life is an act of God’s grace, a gratuitous act of infinite love, by which God descends to man in order to raise him to share in his own life and knowledge and love. In this union man truly shares in the divine mode of knowledge, he knows himself in an identity with God, but he remains distinct in his being. It is an identity, or rather a communion, by knowledge and love, not an identity of being.
https://www.theway.org.uk/back/551Mong.pdf