Pīti

Pīti in Pali is a mental factor associated with the development of jhāna in Buddhist meditation. According to Buddhadasa Bhikkhu, piti is a stimulating, exciting and energizing and dry quality, as opposed to the calmness of sukha.

Definition

Piti is a joyful saṅkhāra associated with no object, so the practitioner is not attaining it by desire. It is often translated into the English word "rapture" and is distinguished from the longer-lasting meditative "joy" or "happiness" which is a subtler feeling which arises alongside pīti.

Mental factor in meditation

In the commentarial tradition on Buddhist meditation, the development of jhāna is described as the development of five mental factors that counteract the five hindrances:

vitakka counteracts sloth and torpor
vicāra counteracts doubt
pīti counteracts ill-will
sukha counteracts restlessness-worry
ekaggata counteracts sensory desire

Both pīti and sukha are born of bodily seclusion and mental quietude in first jhāna, then are born of focused concentration in the second jhāna but only sukha is sustained in the third jhāna while pīti fades away in the course of cultivating pure, mindful equanimity.
The 5th century CE Visuddhimagga distinguishes between pīti and sukha in the following experiential manner:

Fivefold classification

As the meditator experiences tranquillity, one of five kinds of physical pleasure will arise. These are:

Weak rapture only causes piloerection.
Short rapture evocates some thunder "from time to time".
Going down rapture explodes inside the body, like waves.
Exalting rapture "makes the body jump to the sky".
Fulfilling rapture seems to be a huge flood of a mountain stream.

Note that only the last two are considered piti, specifically. The first four are a preparation for the final stage, which is the jhanic factor.