Buddha Abhidhamma - Ultimate Science - BuddhaNet

98
This upekkhà is the state in the middle between karunà
(compassion) and mudità (sympathetic joy). It is this upekkhà (i.e.,
tatramajjhattatà) which is raised to the dignity of a bojjhaïga, one
of the seven factors of Enlightenment. It is different from hedonic
upekkhà which is indifferent feeling.
8 & 9 Kàya-passaddhi and Citta-passaddhi
Here ‘kàya’ does not mean ‘body’; it refers to the ‘group’ of mental
concomitants.
Passaddhi is tranquillity, calmness, quietude or serenity.
So kàya-passaddhi is tranquillity of mental concomitants
whereas citta-passaddhi is tranquillity of citta or consciousness.
From passaddhi to ujjukatà there are 6 pairs of cetasikas which
go by kàya and citta. The reason they were described in pair by
the Buddha is that they occur together and together they oppose
their opponents.
In the case of saddhà, sati, hirã, etc., they are described as
singles because individually they can oppose their opponents.
By citta-passaddhi, only citta is tranquil. By kàya-passaddhi,
not only are the mental concomitants tranquil, but also the body
is tranquil due to the diffusion of wholesome cittaja-råpa, which
is mind-produced corporeality.
Passaddhi has the chief characteristic of suppressing or
allaying of feverishness of passions. It serves as the cool shade
of a tree to a person affected by the sun’s heat,
Passaddhi is opposed to kukkucca (worry). When highly
developed it becomes a factor of Enlightenment (bojjhaïga).
10 & 11 Kàya-lahutà and Citta-lahutà
Lahutà is lightness or buoyancy or agility. Suppressing the
heaviness of the mental concomitants is the chief characteristic