Dhyana is the seventh limb of ashtanga yoga and second limb of Raja Yoga.
Samadhi is the eighth and the last limb of ashtanga yoga and third limb of Raja Yoga.
Samadhi is a state where there is no experience of sense objects; no experience of smell, taste, form, touch and sound.
Raja yoga, samadhi, unmani, manonmani, amarathva, laya, tatva, shunya, asunya, paramapada, amanaska, advaita, niralamba, niranjana, jivanmukthi, sahaja and turiya are the synonymous terms of samadhi.