Steadiness
What is samādhāna? The gathering of the mind to a single point.
What is samādhāna (steadiness)? One-pointedness of the mind.
The mind that will stay
One-pointedness of the mind, the gathering of it to a single thing. This is the last of the six, and in a way it is what the other five were for. Calm settles the mind, restraint settles the senses, withdrawal frees the mind from outward duty, endurance and faith steady it against what would unsettle it; and the mind so readied is a mind that can rest on one thing without sliding off.
It helps to be exact about what is being asked. This is not the trance the yogis call samadhi, where the faculties fall silent and the world drops away. It is plainer and more daily than that: the mind's settled willingness to return to the matter at hand, to the teaching being heard, and to stay with it rather than scatter. What it guards against is the mind that takes in a sentence of truth and at once runs off to a hundred unfinished things, so that the truth is never quite let in.
And so the six end where the inquiry begins. Hearing the truth of the Self asks for a mind that can hold still long enough to receive it. Samadhana is that holding still: not a feat performed once, but a steadiness the seeker keeps coming back to, the quiet ground on which everything still to come is heard.
When your mind is given something true to hear, does it stay with it, or has it already moved on?
In the commentaries
Of the six, this is the last, and it gathers up the rest. Calm quiets the mind and restraint the senses, withdrawal turns the mind from outward work, endurance and faith keep it from flinching; samadhana is the mind that, so prepared, will rest on a single thing and stay there. The precise sense is the mind's settled readiness to come back again and again to the inquiry, not the yogic trance (samadhi) that suspends the faculties. What it guards against is the scattered mind that takes in a sentence of truth and at once runs off to a hundred other things, so that nothing is really received. Steadiness is what samadhana names, and hearing the truth (sravana) asks for it: a mind that can hold still long enough to take it in.