Samapatti is the concentration of the mind on an object. It is a technical term in Ashtanga Yoga of Patanjali. He has defined it as a state of unification of citta (the mind) that has overcome its ksina-vritti (fluctuation) with an object of any one of the three varieties, namely, grahita (the knower), grahana (the sense organ) and grahya (the one to be known) Yoga Sutra I.41).
With reference to these, the state of samapatti is also spoken of as grahitru samapatti. Tanmayata (the state of unification) is described as tadakarata (one assuming the form of the object), or tanmayata (the state of absorption), or of tadanjanata (assumption of the color) or tadrupata (the state of oneness). It is important to note that such a state can be achieved only by a mind in which the fluctuations have become kshina (ineffective).
The process of unification with the object is explained with the example of a gem which shows, due to its purity, the color of any object in its vicinity. How is such a state different from or similar to the state of Samadhi? Patanjali has used the two words interchangeably. But Samadhi is a wider term. It can mean both states: sabija (with object) and nirbija (devoid o any particular object). But samapatti must always have an object on which the mind has to fix itself. Samapatti can never be nirbija (objectless). Samapatti, sabija Samadhi and samprajnata yoga are synonyms. All three words indicate one and the same state, characterized by a gross or subtle object being used for a support (alambana) for steadying the mind.