What makes a retreat a retreat, across the traditions that practice one, is not any single technique but the act of stepping outside ordinary life to make room for a technique to work. A householder who meditates for twenty minutes before work is doing the same mental exercise, in miniature, that the same tradition asks of someone on retreat; what the retreat adds is the removal of everything else, so that the practice is no longer one activity competing with a day's others but the day's only activity.
Theravada Buddhist vipassana retreats, widely taught in the lay meditation centres that spread from Burma and Thailand across the twentieth century, typically run ten days under a fixed schedule of sitting and walking meditation, noble silence observed throughout, no reading, writing or eye contact, so that attention has nowhere else to go. Ignatius of Loyola's Spiritual Exercises, composed in the sixteenth century, structure a retreat of about thirty days, traditionally in full silence, around four weeks of scripted meditations moving a retreatant through sin, the life of Christ, the passion and the resurrection in turn, under the guidance of a director who meets with them daily. A stay at a Hindu ashram is looser in form but shares the same logic, ordinary responsibilities suspended so that study, chanting and meditation under a teacher can occupy the time that would otherwise be divided.
The common thread is architectural rather than doctrinal: each tradition has decided that some depth of practice needs uninterrupted time to reach, and has built an institution, a retreat centre, a monastery guest wing, an ashram, to provide it.