For the DS9 episode of the same name, please see "Equilibrium".
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")