Sadhaka in Advaita Vedanta is a seeker of truth revealed by Vedanta. Upanishads, the Bhagavad Gita, and Brahmasutras (which are known as the prasthana-trayi and form the quintessence of Vedanta) declare that Brahman is the only Reality, the Absolute Reality ‘which is one without a second and is our real nature.” They say that whatever we see is Brahman only. By mistake we see it as multiplicity. An earnest effort for correcting this mistaken vision by change of consciousness is sadhana. One who performs this is a sadhaka.
He tries to transcend the transitory world by realizing it as the manifestation of Brahman. The uninterrupted feeling and the living experience of Brahman within and without is the goal of the sadhaka. This experience of the Absolute Reality behind everything is self realization or God-realization. One who attains this state is a seer, a man of self-realization and a man of knowledge. The sadhaka struggles to reach this state by attaining Brahman consciousness through karma (deed), bhakti (devotion) and jnana (knowledge). A person engaged in such activities without the goal to realize Brahman is not a sadhaka, because spiritual life means brahma (spiritual) consciousness and not merely spiritual activities.
Thus sadhana is a state of spiritual evolution. If the sadhaka is one end of the thread of perfection, a state of God-realization is the other end. The sadhaka is not satisfied by mere information about Brahman, but struggles for realization of Brahman as his own self.