Mudam raati dadati iti 'Mudra.- that which gives happiness. And this happiness reflects in the consciousness.
Unfortunately today the word Mudra is instantly equated to "Hastha Mudras' or Hand gestures, which is not a part of Hatha yoga, but is a part of Tantra shastras. In Hatha yoga, Mudra refers to Psychic gestures, which enable the practitioner to reach closer to the goal of awakening the Kundalini energy. In all there are 10 mudras elaborated in the Hatha Yoga Pradipika, one of the most important literature available on Hatha yoga today. These Mudras should be carefully practiced only under the guidance of a competent guru for it to reap fruit.