This is the ultimate stage of meditation, when the sadhak forgets his individuality and the difference from the object on which he is meditating and gets completely absorbed in the object as if the sadhak and the object of meditation are one.
When one reaches this stage he is able to transcendent the boundaries of time and space and is at a stage where he can be called a Yogi.
The basic meaning of the word Yoga - to join can be understood at this stage. By the mental process the Yogi is able to join his entire individuality - sense organs, mind and soul with the object it is meditating on. Thus, this is the final state of Yoga.
When a person is able to enter into the state of Samadhi on any object, it is able to understand, feel and know that object is totality. He can feel it, touch it, smell it understand its nature and character.
This is how the Vedic Rishies knew, without the use of computers, that once in 4,00,00,000 years all the planets come in a staight line. The time which is termed as the change of Yug. They knew that atoms contained energy which can be converted into light. They could calculate the speed of all the planets and also their distances.