equilibrium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A condition in which all acting influences are canceled by others, resulting in a stable, balanced, or unchanging system.
2. n. Mental or emotional balance; poise.
3. n. Physics The state of a body or physical system at rest or in unaccelerated motion in which the resultant of all forces acting on it is zero and the sum of all torques about any axis is zero.
4. n. Chemistry The state of a chemical reaction in which its forward and reverse reactions occur at equal rates so that the concentration of the reactants and products does not change with time.

Century Dictionary and Cyclopedia

1. n. Equipoise; the state of being equally balanced; a situation of a body in which the forces acting on it balance one another; also, a determination of forces such that they balance one another, so that their resultant vanishes. Thus, when a heavy body rests on a table, the weight and the elastic forces which the weight evokes are in equilibrium (a phrase often used in the Latin form in æquilibrio, or more commonly in equilibrio)—that is, are precisely equal and opposite; thus, a man walking a tight-rope usually carries a pole or balancing-rod to aid him in preserving his equilibrium—that is, in keeping his center of gravity over the rope, so that his weight and the spring of the rope may act in the same vertical line. Similarly, a floating body is in equilibrium when its weight and the upward pressure or buoyancy of the liquid are exactly equal and opposite. When a body, being slightly moved out of its position, always tends to return to its position, the latter is said to be one of stable equilibrium; when a body, on the contrary, once removed, however slightly, from the position of equilibrium, tends to depart from it more and more, like a needle balanced on its point, its position is said to be one of unstable equilibrium; and when a body, being moved more or less from its position of equilibrium, will rest in any of the positions in which it is placed, and is indifferent to any particular position, its equilibrium is said to be neutral or indifferent. A perfect sphere, of uniform material, resting upon a horizontal plane, is in a state of neutral equilibrium; an oblate spheroid with its axis of rotation vertical is in stable equilibrium; while a prolate spheroid with its axis vertical is in unstable equilibrium on the same plane. A body suspended by its center of gravity is in a state of neutral or indifferent equilibrium. If a body is suspended by any other point, it will be in a state of stable equilibrium when its center of gravity is perpendicularly below the point of suspension; but if the center of gravity is above the point of suspension, the equilibrium will be unstable.
2. n. The state of balance of any causes, powers, or motives, so that no effect is produced.
3. n. A state of just poise; a position of due balance. Especially— Mental balance.
4. n. In the fine arts: The just poise or balance of a figure or other object, making it appear to stand firmly. The properly balanced disposition or arrangement of objects, lights, shadows, etc.
5. n. Equality of influence or effect; due or just relationship.
6. n. Indifferent or neutral equilibrium, when the vertical gradient of temperature in still air is exactly equal to the adiabatic rate in moving air, and a disturbed mass stays in its new location.
7. n. Unstable equilibrium, when the vertical gradient of temperature in still air is greater than the adiabatic rate in moving air. In this ease the mass of air when once started in vertical motion continues to rise or fall as the case may be, because the thermodynamic change in its own temperature is less than the change actually existing in the surrounding atmosphere. A thunder-storm, with its ascending currents and formation of tall cumuli, illustrates unstable equilibrium.
8. n. In chem.: An assumed condition of a mass consisting of the same or of different kinds of matter, in which apparently no chemical change is going on, but in which it is imagined that individual atoms are exchanging places with others of exactly similar character, so that in a given (perhaps extremely short) time many molecules may be decomposed and precisely as many molecules, of absolutely similar character, formed.
9. n. The equilibrium of a liquid cooled, out of contact with its solid phase, below the temperature of equilibrium between the liquid and the solid; or of a liquid heated, out of contact with its vapor, above the temperature of equilibrium between the liquid and the vapor having a pressure equal to the actual pressure on the liquid. Water, free from ice, may be cooled many degrees below its usual freezing-point; when brought into contact with a fragment of ice, sometimes when disturbed mechanically, part of the water instantly freezes, and the temperature rises, from that of the labile equilibrium of water alone, to that of the stable equilibrium between water and ice.

Wiktionary

1. n. The condition of a system in which competing influences are balanced, resulting in no net change.
2. n. physics The state of a body at rest or in uniform motion in which the resultant of all forces on it is zero.
3. n. chemistry The state of a reaction in which the rates of the forward and reverse reactions are the same.
4. n. Mental balance.

GNU Webster's 1913

1. n. Equality of weight or force; an equipoise or a state of rest produced by the mutual counteraction of two or more forces.
2. n. A level position; a just poise or balance in respect to an object, so that it remains firm; equipoise.
3. n. A balancing of the mind between motives or reasons, with consequent indecision and doubt.

WordNet 3.0

1. n. a stable situation in which forces cancel one another
2. n. equality of distribution
3. n. a sensory system located in structures of the inner ear that registers the orientation of the head
4. n. a chemical reaction and its reverse proceed at equal rates