translational equilibrium
Translational equilibrium means all the forces on an object are balanced, so it does not speed up, slow down, or change direction. It might be sitting still, or it might be gliding at steady speed in a straight line; either way, nothing about its motion is changing. A book on a table and a plane cruising at constant velocity are both in this state.
An object is in translational equilibrium when the net force on it is zero: F_net = 0. By Newton's second law this means the acceleration is zero, a = 0, so the velocity is constant (which includes being at rest). In practice you write it component by component: the forces in the x direction sum to zero and the forces in the y direction sum to zero, sum(F_x) = 0 and sum(F_y) = 0. ('Translational' refers to straight-line motion of the whole object, as opposed to rotation.)
This is the workhorse condition for solving statics problems: hanging signs, bridges, objects resting on slopes. Whenever something is not accelerating, you know its forces must balance, and setting each direction's forces to zero gives you the equations to find unknown tensions, normal forces, or friction. Full mechanical equilibrium also requires the torques to balance; translational equilibrium is just the force half of that.
A sign hangs at rest from two cords. Because it is in translational equilibrium, the upward pulls of the two cords together exactly balance the sign's weight, and their sideways pulls cancel, letting you solve for each cord's tension.
Zero net force means the forces in each direction separately add to zero.
Constant velocity, not just rest, counts as translational equilibrium; zero acceleration is the real condition. And balanced forces (translational equilibrium) do not guarantee no rotation; full equilibrium also needs the torques to sum to zero.