in the Yogasūtra of Patañjali, Samādhi refers to the eighth limb (aṅga) of Yoga and is a higher level of focalized meditation of the mind, contemplation, absorption in the wholeness or absolute. It”s the ultimate stage of Aṣṭāṅgayoga.
Samadhi Mandir is a construction in the form of Tomb or temple/mausoleum, which can contain the body of a holy man who died in State of Samādhi.