A state of quiet, peaceful stillness.
A state of quiet, peaceful stillness — of either a setting or the mind.
Quiet, peaceful stillness that applies readily to both an environment and a mind — its signature dual reach (“the tranquility of the countryside,” “inner tranquility”), where equanimity and composure are almost exclusively about a person under pressure. More positively serene and stiller than bare calmness; very close to serenity, but tranquility leans toward outward quiet and stillness where serenity leans toward luminous inner peace.
Late 14c., from Latin tranquillitas “quietness, stillness, serenity,” from tranquillus “quiet, calm, still.” (The deeper trans- + “be quiet” analysis is disputed.)
From its late-14c. entry it carried a wide bundle of senses — public/political order and inner peace alike; modern usage has narrowed toward quiet stillness and peace of mind. No reliable recent-generation shift.