Ramana Maharshi describes enlightenment as sahaja nirvikalpa samadhi.
What do these terms mean?
Nirvikalpa samadhi is just an intense concentration state. Ever been intensely focused on something, and everything except that thing blurs into the background? And in fact, that thing kind of disappears too... because your mind is so absorbed in it that it doesn't even notice it's something separate?
That's an intense concentration state. If there is still some effort involved in keeping up the concentration, that means the mind is still flickering... and so it's not exactly samadhi yet. It would be called dhyana -- effortful concentration on something.
When the concentration becomes so intense that you skate onto smooth ice and things become effortless, that's a samadhi state. Samadhi states can sometimes occur accidentally or involuntarily, like when you are shocked, or when you wake up in the morning and exist for a second or two without remembering who or where you are.
When the eyes are closed and the concentration is on something internal, the senses get blocked out... and you get nirvikalpa samadhi.
When the eyes are open and you're involved in something, that same concentration state is called savikalpa samadhi -- or what is also called a state of "flow."
When it's no longer a matter of concentrating on one thing or another, but there is a continuous recognition that underlying any particular thing you can concentrate on, there is something permanent... then that is called sahaja nirvikalpa samadhi (sahaja means natural) That happens eyes open, eyes closed, regardless of activity.
When performing inquiry or surrender, the idea is to create a profound focus on the I (either by inquiring into it, or by ignoring thought, forcing attention away from it and therefore towards the I, which is the only other place it can go). But the I is not an actual object, and if it the focus becomes strong enough, the imagined I, an illusion, gives way to the reality of the Self. In that moment the normal illusory functioning of the I is suppressed. This is sahaja samadhi, but if the attachments draw the mind away from it, that naturalness is not fully established, and the exercise has to be repeated. Slowly the experience of the sweetness of even these glimpses of sahaja nirvikalpa samadhi will draw the power away from the attachments and collapse them, and the true sahaja state will remain unobscured (and in fact one will realize that it has never really been obscured).