Jaku can mean tranquility as in Wa Kei Sei Jaku, the famous principles of tea.

This is a state of mind of oneness, stillness, enlightenment, eternity, happiness, substantialness, pureness, trancendence (of mind-body, object-subject, host-utensil, guest-host, etc.) and a state of complete and harmonious bliss and peacefulness. (From: “Tea and Buddhism” by Ryofu Pussel.)