What is a 'MUDRA'?

A ritualized gesture (or yogic/tantric energy seal) that symbolizes divine powers, and - when practised with focused awareness or intention - alters the energy flow within the body, awakens Kundalini and induces an 'altered' or ecstatic state of consciousness in the practitioner or in those for whom it is performed.

Or:

A spontaneous gesture that expresses a particular energy flow within the body, and an 'altered' or ecstatic state of consciousness (bhava) of the 'practitioner', manifesting either as a purificatory process (kriya) or as a state of grace, and a blessing which may be transmitted to those who witness it, or to those for whom it is performed on a subtle level.

Mudras are found in Yoga, Tantra, Indian dance, and Buddhism, in iconography as well as in spiritual practice.
The literal translation is usually given as 'seal', or 'gesture'.
According to the Gheranda Samhita, the main purpose of mudras is to attain 'steadiness'. According to other texts on Yoga, mudras are especially for awakening Kundalini. Five of the twenty-five mudras given in the Gheranda Samhita are also stated as having the purpose of awakening Kundalini.