Contemplative Glossary

In general usage, samadhi means meditative "absorption," "concentration," or "one-pointedness."  It is the state of mind you reach in meditation when you succeed at remaining in single-minded, undistracted awareness.

Samadhi, a Sanskrit term, literally means "to hold things together."  The Tibetan equivalent, ting ngé dzin, means "to hold firmly and unwaveringly from the depths so that there is no movement." (Per Rigpawiki)

Sometimes the connotation of samadhi stops at mere concentration.  Often, however, it implies deeper states of mind, levels of realization, or nondual wisdom that unfolds within the experience of that meditative absorption.  Also, the word "concentration" sometimes does not fit, as some varieties of samadhi have no specific object of attention, or even no fixed reference points at all.

This is another case where a Buddhist term can have conventional/transcendent or relative/absolute senses of meaning.  Perhaps it is helpful to differentiate between "big S" and "small s" samadhi.

For a deep dive, see Samadhi on Wikipedia.