According to Osho, a seeker is simply the first step of sannyas; a seeker is one who seeks and practices sannyas—transcending pleasure/pain, doership into witnessing, ego into emptiness, matter into oneness with God. A sannyasin is the lived commitment to this journey, and a siddha is its fulfillment; thus seeker to sannyasin to adept, moving toward the immense, the infinite.
A seeker starts the journey, a sannyasin lives it fully, and an adept has completed it.