(Tibetan) Literally, "knowing." The Sanskrit equivilent is vidya. Its opposite is avidya, "unknowing, ignorance." When used as a verb, rigpa means "to be aware, to know." As a noun it means (1) all conscious experiences (knowledge, gnosis), (2) cognizance, (3) the science of Gnosis, (4) pure awareness itself, the very nature of mind.
"Unknowing (marigpa) is to not know the main substance of mind. Knowing (rigpa) is to see this basic substance of mind." - Padamsambhava, The Cycle of Vital Points