我个人也不太懂，应该还是要看上下文，但是我查了下Google，大概有几种解释：
1. Time trend models assume that there is some permanent deterministic pattern across time. These models are best suited to data that are not dominated by random fluctuations.
2. Time-trend designs are a form of longitudinal ecological study, and can provide a dynamic view of a population’s health status. Data are collected from a population over time to look for trends and changes.

这两种解释大概都是讲：随着时间变化，数据显示出一种trend。

c.f.： http://www.healthknowledge.org.uk/public-health-textbook/research-methods/1a-epidemiology/time-trend-analysis