[v] become less tense, rest, or take one's ease; "He relaxed in the hot tub"; "Let's all relax after a hard day's work" [v] decrease the pressure of; "depressurize the cabin in the air plane"
depressurise, depressurize, loosen up, relax, slow down, unbend, unwind
pressurise, pressurize, tense, tense up