統計力學的熱平衡，英文是 thermal equilibrium, 費曼在他的 statistical mechanics 這本書中定義（或描述）平衡時，有一個簡易的說法「 all the fast things have happened, and all the slow things not」，我常用白話文開玩笑說：「該發生的已經發生，不該發生的還沒發生！」，攏統的說，所謂平衡是指「短時間」內，系統狀態沒有明顯變化的意思。

雖然這個定義是用來「含糊」的定義「熱」平衡，但是也可以衍伸來描述所有「平衡」。例如

n ----> p  +   e   +    anti- neutrino

平衡也可指「左邊和右邊的粒子數量（或狀態）沒有明顯的變化」。化學平衡也可「類比」，當然，所有平衡狀態是指往左變化和往右變化的速率相同，導致表面看起來，「短時間」內，兩邊的狀態沒有「明顯」或「可量測」的變化。

According to the definition by Feynman, Ther-mal Equilibrium (TE) is defined as:

• A system is very weakly coupled to a heat bath at a given T,
  
• if the coupling is indefinite or not known precisely,
  
• if the coupling has been on for a very long time,
  
• and if all the fast things have happened, and all the slow things not,
  
  then the system is said to be in TE. 
  

生態平衡

生態平衡指的是一段長時間內在一個生態系統裡面個體，物種和小生境的數目恆定或者是以某一個值作小幅度震動。正是有這樣的震動，人們將這種平衡稱作生態平衡。生態系統發展到成熟的階段，它的結構和功能，包括生物種類的組成、各個種群的數量比例以及能量和物質的輸入、輸出等都處於相對穩定的狀態，這種狀態稱作生態平衡，又稱自然平衡。

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Balance of nature

"Balance of Nature" redirects here. For the TV episode, see Balance of Nature (The Outer Limits).

The balance of nature is a theory that proposes that ecological systems are usually in a stable equilibrium (homeostasis), which is to say that a small change in some particular parameter (the size of a particular population, for example) will be corrected by some negative feedback that will bring the parameter back to its original "point of balance" with the rest of the system. It may apply where populations depend on each other, for example in predator/prey systems, or relationships between herbivores and their food source. It is also sometimes applied to the relationship between the Earth's ecosystem, the composition of the atmosphere, and the world's weather.