Shamatha is the one-pointed
concentration of the mind.
Vipashyana is perfect discernment.
– The Cloud Of Jewels
Because body and mind are interrelated, if one adopts a correct physical posture, the subtle channels with their energies will be straight, and this will facilitate the birth of realisation in the mind. It is therefore important to sit on a comfortable seat and adopt the seven-point posture of Vairochana [Buddha].  The legs should be crossed.  The eyelids should be lowered, and the eyes should be gazing [or closed] along the line of the nose.  The body should be kept straight, not leaning to one side or another.  The shoulders should be level.  The chin should be slightly tucked in so that the nose is in line with the navel.  The tip of the tongue should rest on the palate behind the teeth.  The breath should be allowed to rise and fall slowly and naturally. The posture is referred to as the life-tree or vital axis of the mind, for it helps to prevent it from getting lost in distraction, like tying a feeble sapling to a firm support.
One must then persevere in concentration, keeping the mind balanced in equipoise and focusing it exclusively on the specific object of concentration, without letting it stray elsewhere, even to virtuous objects (let alone negative ones), and without letting it lapse into a vague blankness. The object on which the mind should focus one-pointedly may be with or without form [e.g. the name of Amituofo; Amitabha Buddha], and from time to time, one should through thorough investigation make sure that the mind is focused on its target… When the mind remains in a state without torpor or excitement, dwelling one-pointedly on its object, undisturbed by thoughts, this is ‘calm-abiding’ (shamatha). The perfect recognition of the mind’s nature of primal wisdom or the absence of clinging to the object of concentration is the postmeditation or ‘insight’ (vipashyana). Shamatha and vipashyana partake of the same nature.
– Kangyur Rinpoche
On The Path To Enlightenment: Heart Advice From The Great Tibetan Masters
Compiled By Matthieu Ricard