大过年的，M-W居然给了这么个单词，真不应景。
senescence n. [sih-NESS-unss]
Definition
1: the state of being old: the process of becoming old
1：处于老化的状态或者老化的过程中
2: the growth phase in a plant or plant part (such as a leaf) from full maturity to death
2：植物或植物的某部分（例如一片叶子）的从完全成熟到衰败的生长阶段
Did You Know？
Senescence can be tracked back to Latin senex, meaning "old." Can you guess which other English words come from senex? Senile might (correctly) come to mind, as well as senior. But another one might suprise you: senate. This word for a legislative assembly dates back to ancient Rome, where the Senatus was originally a council of elders composed of the heads of patrician families. There is also the much rarer senectitude, which, like senescence, refers to the state of being old (specifically, to the final stage of the normal life span).
词源/词根演化：senex（old，老）+ -escence （process or state of being，处于...过程中或状态）--> senescence
senescence可以回溯到拉丁语senex（老）。你还能想到一些其他的来自于senex的英文单词么？对，Senile（年老的）可能浮现在你的脑海中，或许还有senior（年长的）。但是还有一个可能会令你吃惊，senate也来自于senex，这个表示legislative assembely（下院）的单词可以追溯到古罗马时代，在那时senatus表示一些来自于贵族家族中的年长的首脑组成的理事委员会。还有一个更罕见的单词senectitude（晚年），与senescence一样，指代处于衰老的状态（尤其指，普通生命过程的最后阶段）
注：senile：年老的，衰老的，年老所致的；patrician：adj. 贵族的，显贵的，n. 贵族；senectitude：晚年
Examples
1: "The results revealed that some trees have shorter or longer sleep period than 12 hours and others show slow continuous movement in one direction probably because of disease or senescence."
--- ScienceDaily, 20 Apr. 2018
结果表明某些树种具有短于或长于12小时的睡眠周期，而其他树种显示出慢性地、持续地单方向移动，或许是由于疾病或者衰老导致的。
2: "Until we're all brain patterns on computers, there are still forces that do not bend to our wants, including senescence and death. (You'll talk like this, too, when you hit 40.)"
--- John Hodgman, The New York Times, 21 Dec. 2108
在我们还没有完全成为运行在计算机中的脑模式之前，某些力量依然不会向我们告饶，包括衰老与死亡。（当你40岁时，你也会这样说的。）。这句话是说，除非我们的脑模式可以下载到计算机中，从而获得永生，否则我们总是要屈服于衰老与死亡的力量之下。
注：bend to：屈服于；want：n. 需求，贫穷，缺乏