A perpetuity is an asset or a contract that pays a fixed amount of money each time period, forever.

永续年金是一种财产或者合同,规定在每一时间段内,支付一定数量的货币,直至永远

耶鲁公开课 - 金融市场课程节选

First, we got to make sure the spring exerts a fixed force every time.

首先,要保证,弹簧每次产生的力是恒定的

耶鲁公开课 - 基础物理课程节选

And fixed versus variable speaks to whether you give a reward on a fixed schedule, every fifth time, or variable, sometimes on the third time, sometimes on the seventh time, and so on.

固定与变化则是相对于,你是否根据固定时程给予奖励而言的，每五次就给予奖励，或者是变化的，有时候在第三次上给予奖励，有时候则在第七次上给予奖励。

耶鲁公开课 - 心理学导论课程节选