Equilibrium was the sense of balance in biology, enabling lifeforms to keep upright when standing or walking.
In 2369, emergency transporter armbands modified to create a subspace isolation field, caused Deanna Troi's equilibrium to get into disorder, which made her feel a little dizzy. (TNG: "Timescape")
In 2376, Tuvok suffered from space sickness, which included the symptoms of gastrointestinal distress and a slight loss of equilibrium. (VOY: "Fair Haven")