Immortal time' is a bias in the analysis of conversion rates, where a period of time is artificially extended due to events that prevent data from being collected, such as customers giving up on using a product or feature. This bias can lead to incorrect conclusions about the effectiveness of certain features or products. By recognizing and adjusting for this bias, analysts can get a more accurate picture of how different features or products impact conversion rates. In the context of Fivetran's analysis of transformation users versus non-users, immortal time was found to be a significant issue that needed to be addressed in order to draw meaningful conclusions about the impact of transformations on conversion rates.