Samadhi is Sanskrit for "complete meditation". According to Vyasa, "Yoga is samadhi" deciphered as complete control (samadhana) over the functions of consciousness. The exact meaning and usage of the term varies among the Indian religious traditions (such as Hinduism and Buddhism) but its meaning is "correct" (samyag) contemplation (adhi) resulting in various degrees of veridical coalescent acquisition of truth (samapatti).
Samadhi is the main subject of the first part of Yoga Sutras called Samadhi-pada.
Samadhi is also the Hindi word for a structure commemorating the dead (tomb).
Mahasamadhi (literally great samadhi) is the Hindi word for a realized yogi's conscious departure from the physical body at death.
In Bhagvad Gita Krishna speaks about Samadhi and about principal stages of Nirvana: Nirvana in Brahman (the Holy Spirit) and Nirvana in Ishvara (the Creator). But in India the term “Nirvana” became widely used by Buddhists at some point in time and later on this term along with Buddhism, was “forced out” from India by Hindus. Instead of using the term “Nirvana” Hindu schools started to expand the meaning of the term “Samadhi” by adding to it various prefixes. Various schools used these composite words and because of this the term “Samadhi” got “diffused” and lost its unambiguity. This is why it makes sense to get back to accurate terminology that God introduced into spiritual culture through Krishna".