1. a correlation between two variables such that the value of one variable is the reciprocal of the value of the other. For example, if a researcher is studying the average time taken to complete a task, then tasks completed per unit time (e.g., 2 per hour) have a reciprocal relationship with unit time taken per task (0.5 hours).

2. the situation in which two variables can mutually influence one another; that is, each can be both a cause and an effect.