Mindfulness (Pali sati; Skt. smṛti; Tib. དྲན་པ་, drenpa, Wyl. dran pa) is one of the fifty-one mental states defined in Abhidharma literature. According to the Compendium of Abhidharma, it belongs to the subgroup of the five object-determining mental states.
Mindfulness is also the fifth antidote of the eight antidotes to the five faults in meditation practice. It’s the antidote to the second fault, forgetting the instructions or the object of focus.
In the Khenjuk, Mipham Rinpoche says:
In terms of shamatha meditation, you could say that mindfulness protects and maintains the 'remaining' or stillness (Tib. གནས་པ་, népa) of mind, so you do not become distracted from it.
In the practice of maintaining discipline, mindfulness is defined as “not forgetting what should be adopted and abandoned.”
In the Mahamudra teachings, there are said to be four kinds of mindfulness: